GENERAL CHARACTERS. 909 



the precoracoid (cl), and the posterior the coracoid (co). At the 

 junction of these three bones there is a cavity for the articulation 

 of the head of the humerus termed the glenoid cavity. Such 

 is the primitive condition of this girdle ; but in Reptiles the pre- 

 coracoid very rarely exists as a separate ossification, although it 

 does so among the Anomodonts, where, at least in the young, 

 it forms a large plate, uniting below to the upper edge of the cora- 

 coid, and entering into the formation of the glenoid cavity. 1 In all 

 other cases it is, however, completely fused either with the scapula 

 or the coracoid. Thus in the Chelonia (fig. 1008) the precoracoid 

 retains its primitive form and condition of a transverse bar, which 

 is, however, completely fused with the scapula. In the Lizards, 

 on the other hand, this bone has united with the coracoid, of which 

 it forms the precoracoidal process (fig. 830, p. cor) — the foramen 

 (for) marking the original line of separation between the two bones. 

 In Dinosaurs and many other Reptiles the precoracoidal process 

 has disappeared, and only the foramen remains ; while in the 

 Ichthyopterygia even this is wanting. A further reduction occurs 

 in the higher Mammals, where the whole of the coracoid has dis- 

 appeared as a distinct bone. The coracoid is subject to great vari- 

 ation in shape, and may either simply meet its fellow by an overlap- 

 ping or sutural junction, or may articulate with the sternum. 



A clavicle, mainly developed from membrane, may be connected 

 with the preaxial borders of the scapula and coracoid ; while a 

 mesial T-shaped interclavicle (fig. 819), which is also developed 

 from membrane, may receive the inner extremities of the two cla- 

 vicles, and then usually overlies the upper part of the sternum. 

 Finally, there may also be a single or double mesial omostemum 

 developed on the ventral aspect from cartilage lying near the an- 

 terior extremity of the girdle. An illustration of the position of this 

 bone is shown in the pectoral girdle of the Frog, represented in fig. 



974- 



In the pelvic girdle we have three separate ossifications arising in 

 cartilage, of which all three usually unite to form an acetabulum 

 (fig. 83 1, 2 a, b) for the reception of the head of the femur, or thigh- 

 bone. The bone commonly known as the haunch-bone forms the 

 dorsal element, and is technically termed the ilium (ibid., II) ; it 

 corresponds to the scapula, and usually articulates with the sacrum 

 by short ribs ; while in the ventral half we have anteriorly the pubis 

 (ibid., P), representing the precoracoid ; and posteriorly the ischium 

 (ibid., Is), which corresponds to the coracoid. The two latter bones 



1 This precoracoid appears to correspond with the bone termed epicoracoid by 

 Professor Cope, and also the one so named in the Monotreme Mammals. 



2 Here the acetabulum is formed by only two bones, and it has been suggested 

 that the bone lettered pubis may be really an epipubis. 



