(112) Columna vertebralis. The total number of vertebrae as well as the number of regional vertebrae varies in different avian taxa. The total number ranges from 39-64 (pygostyle counted as one vertebra). Fewest vertebrae occur in passerine birds; most occur in the swans and ratites. Most interspecific variation in numbers occurs in the cervical series of vertebrae (see Annot. 129). Individual variation in number of vertebrae within taxa is common.

(113) Partes vertebrae. See the review papers of Komarek (1970), and Zweers, et al. (1987) for a detailed treatment of the nomenclature of the features of avian vertebrae. Following Boas (1929) the names of the parts ofa vertebra listed herein are based mainly on a hypothetical "typical" cervical vertebra of Boas' Segment II (see Annot. 129); however, the cervical vertebrae lack distinct, prominent transverse processes such as possessed by the thoracic, synsacral, and caudal vertebrae. Dorsally each vertebra consists of an arch (Arcus vertebrae) and a ventral body (Corpus vertebrae). The opening enclosed by the two is the Foramen vertebrale. Collectively the entire series of the vertebral foramina produce the Canalis vertebralis that houses the spinal cord, its meninges, and the internal vertebral venous sinus (yen. Annot. 46). The arch and body bear several processes which are lever arms for muscle attachment or articular surfaces connecting vertebrae (see Annot. 127a; and Arthr. Annot. 60).

Corpus vertebrae. The Corpus of typical cervical and thoracic vertebrae has expanded cranial and caudal ends, with a constricted midsection, the Concavitas lateralis. The Facies dorsalis corporis (the spinal cord surface of the vertebral body) is not flat, but forms a longitudinal sulcus.

Birds are the only vertebrate animals in which most of the intercorporal articular surfaces are heterocoelus or saddle-shaped (Fig. 4.8). Of infrequent occurence (e.g., penguins, auks, gulls) the vertebrae in the thoracic region are opisthocoelous, having concave caudal articular surfaces (Beddard, 1898). Martin (1987) notes that certain modern birds (e.g., charadriiforms) still retain amphicoelous vertebrae in the region "just anterior to the sacrum" (see Arthr. Annot. 60). The vertebrae of Archaeopteryx and Ichthyomis are amphicoelous, although hesperornithiformes are heterocoelous.

(114) Fovea cranioventralis. Synonymy: Fovea anteroventralis (Boas, 1929). This pit (Fig. 4.8) accomodates the ventral lip of the articular surface of the body of the vertebra cranial to it upon ventral flexion of the neck.

(115) Sulcus lateralis. The groove on the side of the body of a cervical vertebra (Facies lateralis) accommodating the ascending vertebral artery and vein. Tuberositas lig. collateralis. Synonymy: Tuberositas lateralis corporis (Landolt and Zweers, 1985). The caudal end of each vertebral body exhibits on its lateral side a distinct marking for attachment of Lig. collaterale. See Arthr. Annot. 60.

(116) Eminentia costolateralis. Synonymy: Proc. costolateralis (Boas, 1929); parapophysis; Tuberculum costarium (Komarek, 1979). The costolateral eminence is a small prominence of the lateral surface of the bodies of thoracic vertebrae that bears an articular facet, Fovea costalis, for the head of a rib, Capitulum costae. The fovea occurs on free thoracic vertebrae as well as those of the notarium and synsacrum. Replacement of Komarek's term "tuberculum costarium" avoids confusion with the tuberculum of a rib (see below, Annot. 117 and Arthr. Annot. 79.

(117) Fovea costalis. Articular surface on the lateral end of a transverse process of a vertebra for the tubercle of the rib, Tuberculum costae. The costal fovea also occurs on the Eminentia costolateralis (see Annot. 116; and Arthr. 79).

(118) Proc. costalis. Synonymy: Spina laminae ventralis (Komarek, 1970); Pleurapophysis. A rudimentary rib with its proximal end ankylosed to the Corpus and Proc.transversus of a cervical vertebra, its free caudal end forming an attenuated style or spine (Fig. 4.8). See Annot. 141b regarding the costal processes of sacral vertebrae.

(119) Crista [Proc.] ventralis corporis. Synonymy: Hypapophysis; Proc. latus (Boas, 1929); Crista ventralis (Komarek, 1979). These median, ventral crests (processes) display interspecific variablity in shape and relative development.  Crest" is descriptively apt for laterally compressed, plate-like processes. The crests are present on the ventral side of the bodies of the cranial and caudal series of cervical vertebrae, but lacking in the intermediate series (see Annot. 129). The size of the ventral crest on the Atlas of different avian taxa is variable; that of the Axis is quite strong in many birds (see Boas, 1929).

Ventral crests are most strongly developed on the cranial series of thoracic vertebrae and the cervicothoracic transitional vertebrae of spheniscids, Gavia, alcids, and some anseriforms (Beddard, 1898). They are considered adaptations for powerful underwater use of the neck (Kuroda, 1954). See below Annot. 122; and Arthr. Annot. 72. The paired ventral longus colli muscles are attached to the crests (Myol. Annot.56).

Fenestrae intercristales. Synonymy: Foramina intercristales (Komarek, 1979). The ventral crests of cranial thoracic vertebrae (including those of the Notarium) of some taxa are ankylosed to one another. The fenestrae are windows (openings) of variable size and shape where the ventral intercristal ligaments are incompletely ossified, in other words, incomplete fusion of adjacent ventral crests (see Annot. 140); Arthr. Annot. 63).

(120) Alae cristae ventralis. Paired wing-like lateral extensions of the ventral edge of the Crista ventralis; seen in the cranial series of thoracic vertebrae of certain diving birds, e.g., Cavia (Kuroda, 1954); slightly developed in Anas (Landolt and Zweers, 1985) and alcids (Strauch, 1985). According to R. W. Storer (pers. comm.) the alae are best developed in the loons, next in some penguins (Aplenodyles), present in all alcids, and fairly well developed in the larger alcid species (Alca, Uria, Pinguinus, Fratercula) and the diving ducks (e.g., Clangula).

(121) Proc. caroticus. Synonymy: Catapophysis (Beddard, 1898); Proc. sublateralis (Boas, 1929); Proc. hemalis (Komarek, I 970a) . Paired incurved processes on the ventral side of vertebral bodies of the intermediate group of cervical vertebrae (Fig. 4.8B; Annot. 129). The carotid processes are not homologous with the haemal processes of the tail region of birds and other vertebrates (Annot. 144).

Each of the carotid processes forms the lateral wall of the Sulcus caroticus. Slips of M. longus colli ventralis (Myol. Annot. 56) are attached to the carotid processes. In most birds the free ends of a pair of carotid processes are connected by a ligamentous bridge producing a short canal. In certain birds, e.g., Pelecanus, Ardea, Dendrocopos. the paired processes become ankylosed, forming a complete osseous canal (see below). Fused processes are thought to be convergent fearures in species having the ability to throw the head forward (Jenni, 1981). In Dendrocopos the fused carotid processes are equipped with a ventral median crest; Jenni (1981) considers that the crests are adaptations for drilling and drumming (see Annot. 119).

Canalis caroticus cervicaIis. Synonymy: subvertebral canal. On the ventral surface of the intermediate segment of the cervical vertebral column the internal carotid arteries course in this osseo-fibrous canal that is partially formed by the carotid processes (Annot. 121a, 99). See Art. Annot. 15.

Proc. postlateralis (Zusi and Storer, 1969). Synonymy: Proc. inferolateralis (Boas, 1929); Proc. ventrolateral is (Landolt and Zweers, 1985). Seen in ventral view of cervical vertebrae, this process in grebes is a paired caudolateral projection of the vertebral body; for attachment of M. longus colli ventrales (Zusi and Storer, 1969). Present also in Morus and Phoenicopterus.

(122) Crista ventrolateraIis. Synonymy: Proc. inferolateralis (Boas, 1929); Proc. ventrolateral is (NAA, 1979). Ventrolaterally oriented, paired projections attached to the ventrolateral border of the body of certain thoracic vertebra; the ventrolateral crests flank the Crista ventralis on each side; present, e.g., in Larus, and the owls, Strix, Nyctea. See Fig. 4.8A.

(123) Proc. transversus vertebrae. Synonymy: Diapophysis. Paired process that projects laterally from each side of the vertebral arch. During postnatal maruration of the skeleton the transverse processes of cervical vertebrae become fused with cervical ribs (see Annot. 134, 138). The transverse process of most of the cervical vertebrae is not a pronounced fearure as in the thoracic, synsacral, and caudal regions (see Annot. 134, 135); it is often indistinguishable from the Ansa costotransversaria of cervical vertebrae in marure birds (Annot. 135).

(124) Crista transverso-obliqua (Boas, 1929). The cervical vertebrae of longnecked birds best exhibit this crest on the dorsal surface of the vertebral arch. The crest of each side extends obliquely caudolateralIy onto its caudal zygapophysis.

(125) Torus dorsalis. Synonymy: Hyperapophysis (Beddard, 1898); Processus dorsalis (Boas, 1929). This boss of bone is found on the Crista transversc-obliqua of the dorsum of the caudal zygapophysis (Fig. 4.80); for attachment of Mm. ascendentes (see Myol. Annot. 46-49). The location of the torus varies from the base to near the tip of the zygapophysis; it is strongly developed on cervical vertebrae of some forms (e.g., Alca, Haliaetus, Morus). The use of "Torus" is preferable since it avoids confusion with the spinous [dorsal] process of the vertebral arch.

(126) Area lig. elastici. Synonymy: Facies lig. e1astici (Komarek, 1970). Interlaminar and interspinous elastic ligaments are usually ·attached cranially and caudally on the dorsal lamina of the vertebral arch at the base of Proc. spinosus [dorsalis]. Bony markings of the ligaments are variously developed as roughened ruberosities, facies, fossae, or foveae that are here designated generically as "areae". See Arthr. Annot. 63.

(127a) Arcus vertebrae. Each end of the vertebral arch is attached to the dorsolateral border of its vertebral body; the arch forms the lateral wall (Lamina lateral is arcus) and the dorsal wall (Lamina dorsalis arcus) of the vertebral canal (see below). The transverse process is a lateral projection of the arch;  ts base marks the dividing line between lateral lamina and dorsal lamina, best exhibited in thoracic vertebrae since cervical vertebrae lack prominent transverse processes. The level of the zygapophyses indicates the dividing line between dorsal and lateral laminae in cervical vertebrae.

Lamina lateralis arcus [Pediculus arcus]. Lamina lateralis is a substitute name for the mammalian "Pediculus". In birds the lateral part of the vertebral arch is plate-like rather than a constricted stalk (pedicle) as in mammals; this lamina is especially . expansive in the "long vertebrae" of birds (Komarek, 1970a). See Annot. 128a.

Lamina dorsalis arcus is the segment of the vertebral arch that extends from the base of the transverse process of one side to that of the opposite side; it bears the Proc. spinosus on the midline of its dorsal aspect. See Annot. 128 b, c.

(127b) Lamina arcocostalis (Landolt and Zweers, 1985). This lamina is a thin shelf of bone continuous with the caudal margin of Ansa costotransversaria (Fig. 4.8D); the lamina extends lateroventrad from the vertebral arch often over the entire length of the spine of the costal process in anserids and anatids. In Gallus and Phoenicopterus the arcocostal lamina is less extensive, not reaching the tip of the costal process. The lamina may be considered as an extension of the Ansa which forms the dorsolateral wall of a craniocaudally-attenuated transverse foramen whereby the foramen becomes converted into a canal (see Annot. l27c).

(127c) Lamina corporocostalis (new term; well illlustrated, but not named by Komarek, 1979: 106). In conjunction with the occurrence of the arcocostal lamina, another lamina, the corporocostal lamina, extends medially from the costal spine to the vertebral body. It forms the ventral floor of the attenuated transverse foramen (canal) in the birds noted in the paragraph above. Both the arca- and corporocostal laminae are derived by ossification of intermuscular aponeuroses or fascial sheaths (see Myol.).

(128a) Incisura caudalis/cranialis arcus. Synonymy: Incisura vertebralis (Komarek, 1979). These are notches in the cranial and caudal borders of the Lamina lateralis arcus. The cranial notch of one vertebra and the caudal notch of the vertebra in front of it together form the boundaries of a Foramen intervertebrale for passage of the spinal nerve and vessels into and out of the vertebral canal. The caudal incisure is generally markedly the deeper of the two (Annot. 143a).

(l28b) Hiatus interarcualis. The opening or gap between the dorsal laminae of the arches of adjacent (articulated) vertebrae as seen in dorsal view (see Zusi, 1962; Komarek, 1979). The hiatus is closed by the interlaminar elastic ligaments and membranes (see Arthr.). The hiatus is bounded by the Lacunae interzygapophysiales of the dorsal laminae of the arches of two adjoining vertebrae (see below), most pronounced in the cervical region (see below).

(128c) Lacuna interzygapophysialis (new term). "lncisura arcualis" (Komarek, 1970), has been replaced to avoid confusion with the lncisurae cranialis/caudalis arcus (of Lamina lateralis) that are boundaries of the intervertebral foramina. The Lacuna is the V-shaped or often broadly V-shaped indentation of the Lamina dorsalis of the vertebral arch, located between the right and left zygapophyses (Fig. 4.8C) at each end of a vertebra (see paragraph above); two adjoining lacunae form the cranial and caudal boundaries of the Hiatus interarcualis.

(129) Vertebrae cervicales. The greatest number of cervical vertebrae are found in ratites (ca. 20) and in swans (23-25); fewest in coraciiforms and passeriforms. Boas (1929) characterized the cervical vertebral column as consisting of three morphologically and functionally distinct sections: Segment I, the most cranial series, Segment II, the intermediate series, and Segment ill, the most caudal series. Zusi (1962) noted that the joints within and between the segments permit I and III to be flexed ventrally, but Segment II can be flexed only dorsally; this arrangement allows the neck to be held in its characteristic S-shaped retracted position. In birds generally, most of the cervical vertebrae are invaded by diverticula of the cervical system of air sacs (see below); however Boas (1929) reported that all of the cervical vertebrae were apneumatic in the following diving birds: Colymbus (Gavia) , Plotus (Anhinga) , Podiceps, Aica, and Spheniscus.

(130) Atlas; Axis. These are the specialized first and second cervical vertebrae, respectively. The Axis is also known as Epistropheus. The Atlas is apneumatic in all birds examined, the Axis being apneumatic in many birds (Boas, 1929).

(131) Fossa condyloidea. Synonymy: ventral semi-ring (Boas, 1929). Cupped-shaped or semicircular concave surface of the Atlas for articulation with the occipital condyle of the base of the skull.

Incisura fossae; Foramen fossae. The condyloid fossa on the cranial aspect of the Atlas may be perforated (Foramen fossae) or have an open dorsal notch (Incisura fossae) in which the apex of the dens rides. See Arthr. Fibrocartilago atlantis.

(132) Zygapophysis caudalis [Proc. articularis caudalis]. Synonymy: postzygapophysis. Zygapophysis cranialis [Proc. articularis cranialis]. Synonymy: prezygapophysis. The zygapophysis is one of four processes of each vertebra that project from the vertebral arch or the base of the transverse process. The pair of cranial zygapophyses of one vertebra and the pair of caudal zygapophyses of the vertebra in front form freely moveable synovial joints on each side. The free caudal vertebrae of most birds lack zygapophyses; exception: the albatross Diomedea (see Annot. 128c; Arthr. Annot. 65).

Caudal zygapophyses are present on the Atlas of most birds studied by Boas (1929), thus paired atlantoaxial zygapophysial articulations exist. See Arthr. Annot. 68.

(133) Proc. costalis atlantis. A rudimentary rib is not evident on the Atlas of most birds (Boas, 1929); therefore the Atlas of relatively few birds exhibits transverse foramina (see Annot. 134, 135). Boas (1929) depicted well developed, complete transverse foramina of the Atlas in Rhea and Cygnus, incomplete ones in other forms. See Annot. 118, 123, 134, 138.

(134a) Foramen transversarium. Synonymy: Foramen costotransversarium. The transverse foramen characterizes most of the cervical vertebrae of birds (for exception, see Annot. 133). Even though the avian transverse foramina may have considerable length and might be referred to as canals (Annot. l27b, c), the term "Foramen transversarium" is retained for consistency with the mammalian nomenclatures. See below, Annot. 135 for the fetal derivation of the foramen.

(134b) Canalis vertebrarterialis (Boas, 1929). On each side of the cervical vertebral column the series of transverse foramina forms this canal that extends the length of the cervical column and conducts the ascending vertebral artery and companion vein(s) (Art. Annot. II).

The cervical transverse foramen is the equivalent of the opening formed in the angle between the tuberculum and capitulum of each rib and the transverse process of a thoracic vertebra (see Fig. 4.8A, B; Annot. 148). Thus the series of thoracic costovertebral openings is morphologically equivalent to the cervical vertebrarterial canal; moreover, it carries the descending vertebral vessels (Art. Annot. 11) as well as loops of the paravertebral autonomic nerve trunk.

(135) Ansa costotransversaria (Boas, 1929). Synonymy: Lamina ventralis (Komarek, 1970). The Ansa (L. loop) is formed by postnatal ankylosis of the rudimentary cervical rib (Costa cervicalis) with the transverse process and vertebral body of a vertebra. Thus the ansa represents part of the external wall of a transverse foramen, and the body (and lateral lamina of the vertebral arch) form the medial wall (Fig. 4.8B) The ansa demonstrates suiface features: a knob-like Tuberculum ansae (Knopffortsatz, Boas, 1929) and a series of linear Cristae laterales (Langskanten,  Boas, 1929). These features mark the attachment of tendons of lateral cervical musculature (Myol. Annot. 53, 54).

(136a) Incisura caudalis arcus. The caudal notch of the arch of the Atlas forms the rostral boundary of the atlanto-axial intervertebral foramen for the second cervical spinal nerve (see Annot. 128a).

(136b) Dens axis. Synonymy: Proc. odontoideus. The joints between the avian Axis and Atlas differ from those of 'mammals: in addition to the articulation of the Dens with the Atlas, an Artc. intercorporea and paired zygapophysial articulations are present. The atlas and axis are ankylosed in adult hornbills (Bucerotidae) (Kemp, 1985).

(137) Proc. spinosus [P. dorsalis] axis. Although commonly present, the spinous process is lacking from the Axis of some forms (e. g., the scolapacid, Gallinago delicata).

(138) Proc. costalis axis. The rudiment of a rib is present on the Axis of many, but not all, birds that have been studied; occasionally weak projecting tips of the costal processes are found (Boas, 1929). When present, Proc. costalis forms an arch and completes the transverse foramen. See Annot. 133, 135.

(139) Vertebrae thoracicae. Synonymy: Vertebrae dorsales. The first thoracic vertebra is defined as the cranialmost vertebra with a complete rib (i.e., having vertebral and sternal segments) that articulates directly or indirectly with the sternum (see Annot. 147). The vertebrae at the root of the neck that bear moveable ribs, notreaching the sternum, have been called "Vertebrae cervicodorsales" (Newton, 1896;Zusi, 1962); these are transitional in configuration between cervical and thoracic vertebrae. See Annot. 141a; and Arthr. Fig. 5.10).

(140a) Notarium. Synonymy: Os dorsale. The Notarium (Ok. noton, back) is a unit of several (2-6) (Barkow, 1856; Storer, 1982) thoracic vertebrae that are coalesced rather completely in adults, but not fused with the synsacrum (see Arthr. for significance of the joint between the notarium and synsacrum). The Notarium (Fig. 4.9) is characteristically present in at least 17 families of birds, occasional in several others: tinamous, Pelecanus. threskiomithids (ibis and spoonbills), galliforms, columbiforms, as well as all podicipediforms and most falconids (Storer, 1982). The Mesozoic birds Archaeopteryx and Gobipteryx possess several "fused anterior dorsal (thoracic) vertebrae" (Martin, 1987).

In certain birds (e.g., larids, rhynchopids, gruids, Branta and Anser) consolidation of the thoracic vertebral column is achieved by ossification or calcification of the epaxial muscle tendons that interdigitate and may fuse to one another and to the transverse and spinous processes of adjacent vertebrae. This sort of consolidation as well as the rather complete synostotic coalescence (above) are both found in some groups (e.g., grebes and cranes; R. W. Storer, pers. comm.). See Arthr. Annot. 71.

(l40b) Canalis notarii. The segment of the vertebral canal that traverses the Notarium (see Annot. 144). Crista spinosa [dorsalis] notarii. Synonymy: Crista dorsalis notarii (NAA, 1979). Crest formed by the ankylosed spinus processes.

(141a) Synsacrum. Synonymy: Os lumbosacrale; Os pelvicum. A rigid unit consisting of ankylosed vertebrae in mature birds (Figs. 4.9, 11). The preacetabular part of the Synsacrum incorporates one or several thoracic vertebrae and the "lumbar series" (synsacral segment II of Boas, 1933) that are attached to the preacetabular ilium: the proper sacral vertebrae are opposite the acetabulum (see below); several more of the proximal caudal vertebrde caudales (urocaudals, Parker. 1888) comprise the postacetabular series. Interspecific variation exists in the number of vertebrae forming the synsacrum. See Barkow (1856), Boas, (1933), and van Oort (1905) for detailed comparative studies of the synsacrum in different taxa.

The synsacrum is synostosed on each side with the Os coxae, the three elements forming the bony pelvis; the pelvis and uropygium (fopog. Annot. 36) together form the dorsal abdominal wall (Baumel, 1988). See Arthr. Artcc. synsacri.

(141b) Vertebrae sacrales. One or two "true" sacral vertebrae (Segment III vertebrae of Boas, 1933) are identified by their conspicuous costal processes, lacking in the vertebrae to the front and rear of them. In some birds the costal processes of the sacral vertebra(e) extend laterally to the hip bone near the acetabulum, thus the name, Vertebra acetetabularis (Du Toit, 1912-13; Komarek, 1979; Radu, 1975) which is well exemplified in the pelvis of Larus, Stm, Gallinula. See Fig. 4.9.

(141c) Lamina transversa notarii/synsacri. During skeletal maturation the transverse processes of the notarial and synsacral vertebrae become coalesced, producing on each side a continuous transverse lamina. In mature birds the lateral border of each Lamina of the synsacrum becomes firmly ankylosed with the hip bone (Os coxae) of its side. In instances where the fusion between the transverse processes is incomplete, the persistent windows are known as Fenestrae intertransversariae. The fenestrae as well as smaller foramina are traversed by nerves and vessels. SeeFigs. 4.9, 11.

(142a) Corpus notarii/synsacri. This is the unit of consolidated vertebral bodies (corpora) that form the median, ventral column of bone of the notarium and that of the synsacrum.

142b) Facies visceralis synsacri. Synonymy: Facies abdominalis. See Barkow (1856) and Boas (1933) for features of this ventral (internal) surface of the synsacrum which is in contact with abdominal organs (viscera).

(142c) Crista spinosa [dorsalis] synsacri. The crest formed by the ankylosed spinous processes of the synsacral vertebrae. .

(143a) Foramina intervertebralia. Dual intervertebral foramina may exist in some birds over part of the length of the synsacrum, especially immature ones; these are separate openings for the dorsal and ventral roots of the spinal nerve, the roots uniting external to the vertebral canal (e.g., Struthio, Rhea, Somateria, Porphyrio, Alca, Corvus) (Boas, 1933).

(143b) Canalis synsacri [vertebralis]. See Annot. 140b. The part of the vertebral canal of the synsacrum. The canal is enlarged along the middle of its length; the enlarged chamber contains the lumbosacral intumescence of the spinal cord which is known as the Cranium inferior (or ischiadicus) by older authors (Barkow, 1856).

(144) Proc. haemalis. Synonymy: intercentrum; chevron bones. Found only on the rear three or so caudal vertebrae on their ventral surfaces, including the pygostyle. Prominent in large birds (e.g., albatross, penguin, heron, pelican) and in some smaller forms (e.g., Crotophaga, Dendrocopos); inconspicuous and vestigial, e.g., in the pigeon and chicken. The haemal processes are usually ankylosed to the cranial ends of the vertebral bodies, projecting ventrocranially and underlying the intervertebral discs and rear of the vertebra ahead. In Crotophaga the processes are fused at their bases with the vertebral body, and also articulate firmly with the body of the vertebra cranial to it.

In some mature birds certain of the haemal processes occur as distinct nodular elements artached by ligaments to the discs and/or to the vertebral bodies (Diomedea sp.). The haemal processes are persistent intercentra (Piiper, 1928), an element of embryonic vertebrae; absent in other vertebral regions except the atlas and axis. Archosaurs closest to birds lack intercentra in the vertebral column except in the tail and CI, C2. See Annot. 121 for comparison with cervical carotid processes.

(145) Pygostylus. Synonymy: Urostylus; Coccyx. Compound bone formed by postnatal ankylosis of 3-6, commonly 5-6, of the terminal free caudal vertebrae. The fetal development of the pygostyle is reviewed by Steiner (1938) and van Oort (1905). Holmgren (1955) contended that the Ostrich pygostyle is not homologous with that of carinate birds, a claim refuted by de Beer (1956). See Baumel (1988) for the structures artached to the pygostyle, its relationships, and remarks on its evolutionary significance.

(146) Basis pygostyli. Derived from fusion of the several vertebral bodies incorporated into the pygostyle. Lamina pygostyli. Blade-like portion of pygostyle derived from vertebral spinous processes and arches. Rudimentary transverse processes are present on the pygostyle of certain piciforms (Burt, 1930: 478). Discus pygostyli: In woodpeckers (piciforms) especially, and other scansorial birds, the 'pygostyle is distinguished by a strong transverse, shield-like disc on its caudal margin (Burt, 1930), the disc serving as an expanded area of attachment for the extraordinarily well developed muscles that depress the tail.

(147) Costae. The freely moveable ribs of different avian taxa vary in number. Ribs of the cervicothoracic transitional region of the vertebral column are short "floating ribs" that fail to reach the sternum (Costae incompletae). The so-called "true ribs" (Costae completae verae) consist of vertebral and sternal elements; the sternal segments articulate with Margo costal is stemi (see Annot. 157). In some instances the sternal part of one or more of the ribs do not articulate directly with the sternum (Costae completae spuriae), but with the sternal parts of true complete ribs cranial to them. Caudal to the true ribs a variable number of floating vertebral ribs may occur; the last of the series of true ribs often articulates with the ventral side of the preacetabular ilium in various birds (Artbr. Annot. 80).

(148) Proc. uncinatus. Synonymy: Appendix epipleuralis (Shufeldt, 1890). Dorsocaudally oriented process attached to the caudal border of the vertebral ribs. Screamers (Anhirnidae) and megapodids lack uncinate processes (R. W. Storer, pers. comm.). See Myol. Annot. 59.

Incisura capitulotubercularis. The neck region (collum) of a vertebral rib exhibits this notch between its capitulum and tubercle. The interval between the neck and the transverse process of the vertebral rib corresponds to the transverse foramen of the cervical vertebrae. See Fig. 4.9; Annot. 134.

Sulcus pulmonalis. The elongated sulcus between the dorsal parts of adjacent ribs. The sulcus is occupied by lung tissue, the Torus intercostalis (Resp. Annot. 49); each Torus is in contact with the ribs cranial and caudal to it, as well as with the intercostal muscles and parietal pleura.