704 COMPARATIVE ANATOMY 



the clavicle is entirely lost. In limbless lizards, the girdles are greatly 

 reduced, and in fact in the Ophidians the girdle itself has completely 

 vanished. 



In birds (Fig. 418), the scapula is formed as a sword-shaped bar 

 overlying the ribs, while the coracoid extends from the glenoid fossa to 

 the anterior end of the sternum. The procoracoid has entirely disap- 

 peared. The two clavicles unite ventrally to form the wishbone, called 

 the furcula (Fig. 418, B.cl.). This may either articulate with the ster- 

 num or lie free. 



In the monotromes the shoulder girdle is quite like that of the lizard. 

 This is also true of the young marsupials, but in the adult it becomes 

 quite like that in all other adult mammals. The coracoid in this instance 

 is reduced to the small coracoid process, definitely ankylosed to the ven- 

 tral end of the scapula. The scapula is well developed, forming a crest 

 called the spina scapulae on its external surface, which in turn culminates 

 in an acromion process (Fig. 416, II, E, acr.). The clavicle varies with 

 the manner in which the limb is used. 



In the higher forms of mammals, the clavicle serves as a strong 

 brace between shoulder and sternum. However, in the ungulates, in the 

 whales, and in a few carnivores it has entirely disappeared. In some 

 mammals it appears as a mere rudiment, without apparent functional 

 value. 



Two small, cartilaginous elements often intervene between clavicle 

 and sternum, called episternalia, or suprasternalia. Their homologies 

 are unknown. 



THE HIP GIRDLE 



The hip or pelvic girdle (Fig. 419) is quite homologous to the shoul- 

 der girdle, the acetabulum representing the glenoid fossa. The ilium 

 represents the scapula, while pubis and ischium represent the procora- 

 coid and coracoid. The gap or open space between pubis and ischium 

 is known as the ischio-pubic fenestra. In the lower forms there is an- 

 other opening, called the obturator foramen, through which the obturator 

 nerve passes to the pelvis. In the higher forms, this usually unites with 

 the ischio-pubic fenestra, the entire opening then being called the obtura- 

 tor foramen. 



In the lower forms, such as the fishes, the basalia are on the inside, 

 and fused to form a single basal, through which the obturator nerve may 

 pass. The radialia are on its distal surface. The basalia of the two sides 

 do not meet, though there is often a small (or a pair of small) cartilage 

 plates between them. These are supposed to be the homologues of the 

 epipubis of the higher forms. There is no acetabular joint. 



In the ganoids and teleosts, ossification begins, but there are no epi- 

 pubic elements. The pelvic fins may migrate so as to lie in front of the 

 pectoral. 



