The clavicular process of the coracoid gives attachment in Carinatae to the deltoideus minor muscle and principal ligament of the shoulder-joint. In Ratitae it is represented by a mere roughness or slight tuberosity. It must be considered as a process developed for the same reason and for the same purpose as the deltoid tubercle or supinator ridge in the humerus of some Mammalia, e.g. Armadillo. The structures attached to it are of prime importance in flight. The two clavicles ossify parosteally. They may be absent, as in all Ratitae save the Emu, and in some Parrots; or fail to meet ventrally - Emu, Toucan, some Parrots and Owls. The upper end (epicleidium), small in the Pigeon, may be large, e. g. Goose, and is stated to be in this case cartilaginous in the embryo. The hypocleidium is small in the Pigeon; it is large and directed downwards, e. g. Fowl, or backwards, e.g. Rook. A coraco-clavicular ligament unites each clavicle to the inner border of the coracoid, and a sterno-clavicular ligament unites the hypocleidium to the carina.
Irregular ossifications may appear in these membranes.
The proportions of the segments of the fore-limb one to the other vary much. In Ratitae they show scarcely a trace of the characteristic Avian position when at rest. The Cretaceous Hesperornis has only the humerus, and the limb is either absent or reduced in the extinct Dinornithidae. In Archaeopteryx the parts of the hand are free, and each digit appears to have borne a claw. A carpo-metacarpus exists in all other birds. In the embryo fowl a mass of cartilage (=carpalia i + ii) corresponds to the two first metacarpals, and a second (=carpalia iii+ iv) to the third and embryonic fourth metacarpals. Uria grylle has, according to Morse, embryonic claws to the first and third digits.
The bones of the pelvis are separate inter se in Archaeopteryx. In all other birds they fuse, at least in the acetabulum. The real length of the ilium is to be measured from the outer ends of the sacral ribs to the acetabulum. The great extension of the bone along the sacrum represents breadth, and is an exaggeration of a Crocodilian and Deinosauran feature. The praeacetabular section, with few exceptions, ossifies parosteally, and it is rarely shorter than the postacetabular section, as in the Ostrich, Divers, and Ichthyornis. The length corresponds with the iliac axis of Professor Huxley. As in all Sauropsida this axis makes an acute angle forwards, not backwards as in Mammalia, with the sacral axis, i. e. a line drawn antero-posteriorly through the centra of the sacral vertebrae. The dorsal iliac area is little developed in some birds, e. g. Ostrich, Divers.
The backward inclination of the pubes and ischia, and the loss of the ventral symphysis is characteristic of birds among living Vertebrata. So far as the pubes are concerned, there was no symphysis in many of the extinct Deinosauria, and the same is true in some instances of the ischia also; and this is especially the case in the two groups Stegosauria and Ornithopoda, in which the conformation of the pelvis is Avian and the ischia with the main part of the pubes (post-pubes) are inclined backwards. The Ostrich is the sole example of a bird with a pubic symphysis, and in Rhea the ischia meet not ventrally, however, but dorsally immediately under the backbone. The union between the distal ends of the pubis and ischium of the same side, and of the latter with the ileum, does not always occur, e. g. Cassowary, Emu, Apteryx among Ratitae; the Tinnamou among Carinatae. There is a well-developed pectineal process in front of the acetabulum in the Ostrich. An examination of a young specimen shows that both ileum and pubis enter into its formation.
A similar process is found in some Carinatae; but it appears to be formed entirely by the ileum in the Fowl. The chick, however, as proved by Miss A. Johnson, has at an early period a large forward extension of the pubis which gradually dwindles away. This process appears to be the homologue of the prae-pubis (so-called) in the Stegosauria and Ornitho-poda among Deinosauria, whilst the main portion of the bird's pubis is the homologue of the post-pubis (so-called) in the same groups. There does not appear to be much ground for supposing, (as has been done) that the prae- and post-pubis represent separate bones. They are continuous one with the other in Stegosaurus (cf. Marsh, American Journal of Science and Arts, xxi. p. 169, PI. viii.), in Camptonotus (op. cit. xviii. p. 502), Laosaurus (op. cit. xvi. p. 415, Pl. x.), and in Iguanodon (Dollo, Bull. Mus. Roy. d'histoire Naturelle de Belgique, ii. 1883, Pl. iii.). And the solitary instance of Allosaurus, which was supposed by Marsh to have had a separate post-pubis (American Journal cited, xvii. p. 90, Pl. vii. 2), is now included by that author in a group of Carnivorous Deinosauria, the Theropoda, in which the post-pubis is absent.
See Marsh on Theropoda, American Journal of Science, xxvii. 1884. Baur has recently suggested that the pectineal process (in part) of Birds and of Deinosauria, or the prae-pubis in some of the latter group, is the homologue of the Os acetabuli of Mammalia (see note, p. 107, Z. A. ix. 1886). For the Os acetabuli, see Gegenbaur, Ausschluss des Schambeins von der Pfanne, etc., M.J. ii. 1876; Krause, Centralblatt fur Median. Wissenschaften, 1876, p. 817; Leche, Bronn's Klass. und Ordnungen des Thierreichs, vi. pt. 5, Mammalia, p. 576; Id. Monthly Internat. Journal of Anat.
And Histology, i. p. 363. For a figure of Iguanodon, see Moseley, Nature, xxviii. 1883, p. 441; or Dollo, op. cit. supra, Pl. v. Cf. Hulke, Journal Geol. Soc. xl. 1884, p. 53; and Huxley, Anatomy of Vertebrated Animals, 1871, pp. 223-228.
The process of the ischium which divides the obturator foramen into two portions is very large in the Deinosaur Laosaurus. The upper division of the foramen transmits in birds the tendon of the obturator internus muscle, a rather curious point. Of the segments of the hind-limb, the femur is remarkably short and very broad in Hesperornis and the Divers; while the tibio-tarsus is of great length in Ratitae and Waders. The fibula is as long as the tibia in Archaeopteryx. It is so at one time in developing birds but shortens subsequently. The tarsal element of the tibio-tarsus has been found by Baur to appear in the embryo chick as two bones, a tibiale and a fibulare, corresponding to the tibia and fibula respectively. The tibiale develops an ascending anterior spur, while the tibia broadens out so as to cover the fibulare, and the fibula itself shortens. The two tarsal bones subsequently unite. According to Morse the ascending process corresponds in the embryoes of certain birds representing various groups (Alcidae, Laridae, etc.) to a separate intermedium. If this is so, there are three proximal tarsalia in some birds, each with its own ossific centre.