SKELETON. I 09 



scapula, where it often forms a part of the glenoid fossa. The scapula 

 is always well developed, and in the placental mammals bears a strong 

 crest (spina scapulae) on its external surface, terminating ventrally in 

 an acromion process. The clavicle varies with the freedom of motion 

 of the limb. Thus in rodents, insectivores, bats, some marsupials and 

 the higher primates it forms a strong brace between shoulder and ster- 

 num. In ungulates, whales, and a few carnivores it has entirely dis- 

 appeared, while in other mammals it persists as a rudiment without 

 functional value. In development two small elements frequently 

 intervene between the clavicles and the sternum (fig. 55). They 

 are preformed in cartilage but eventually fuse with the sternum. 

 Their homology is very uncertain. They have been called episternalia, 

 suprasternalia, etc. 



The Pelvic Girdle (Pelvis}. 



In its broader features the pelvis (cf. fig. 108) is much like the 

 shoulder girdle, and in its full development, may be compared, part by 

 part, with the anterior arch. Thus the acetabulum or socket where the 

 appendage is attached, is comparable to the glenoid fossa. Dorsal 

 to this is the ilium in the position of the scapula, while ventral and 

 medial to the acetabulum are, on either side, an os pubis in front, an 

 ischium behind, with a gap (ischio-pubic fenestra) between them, just 

 as between coracoid and procoracoid. An important landmark is 

 the point of passage of the obturator nerve through the pelvis. This 

 may have its own (obturator) foramen, though the pubic portion or 

 the foramen may unite with the fenestra, the condition in the mammals 

 where the common opening is called the obturator foramen. 



The phylogenetic history of the pelvis is more clearly indicated than 

 is that of the pectoral girdle, for in many fossils, as well as in the 

 sturgeon, there is little advance over Cladoselache (p. 104). The 

 basalia of a side have fused to a single basal, often perforated for the 

 obturator nerve, and bearing the radialia on its distal surface. The 

 basalia of the two sides have not met, but there is frequently between 

 them a pair of small cartilage plates, possibly the homologues of the 

 epipubis of the tetrapoda (infra). There is no acetabular joint. In 

 the other ganoids and in teleosts there is little advance, aside from ossifi- 

 cation of parts, while no epipubic elements occur. A noticeable 

 feature in many acanthopterygians is the forward migration of the pelvic 



