SKELETON. 10g 
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 scapulz) 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 (¢f. 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 (imfra). 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 
