262 



VERTEBRATE SKELETON 



does a fourth bone, the acetabular. All three meet as a rule in the 

 acetabulum which is cup-shaped, but occasionally the pubis is 

 excluded, the cup then being formed by iHum and ischium, the 

 acetabular bone sometimes participating. The ilium is connected 

 with the vertebral column in all cases where the development is 

 known, by sacral ribs, the connexion varying in firmness, being effected 

 by ligament, cartilage, or actual fusion. Usually the pubes and 

 ischia of the two sides meet below in a median symphysis, the ischia 

 faiUng to meet more often than the pubes. Occasionally all sym- 

 physial sutures and those of the acetabular region are obliterated in 

 the adult. 



AMPHIBIA. — The Aistopod Stegocephals, Gymnophiona and 

 Siren lack a pelvis ; the Anuran pelvis is greatly modified in correla- 

 tion with the leaping habits. 



Stegocephala (fig. 279) have only the bones preserved, but their distances 

 from each other and their relations are such as to warrant the conclusion that 



there was a large amount of cartilage present 

 which has left no trace. All three elements are 

 ossified in part, and the pair of ischia, larger 

 than the pubes, are frequently united in a sym- 

 physis. The short and stout ilium is connected 

 with the single sacral vertebra. 



Fig. 279. — Pelvis of Disco- 

 saiirus (Credner). il, ilium; is, 

 ischium; p, pubis. 



Urodela. — In these Amphibia the 

 cartilages of the two sides grow around 

 the obturator nerve and fuse in the middle 

 line, forming a large ischio-pubic plate with the obturator foramen in 

 front of the acetabulum. Later, there is an ischiadic ossification on 

 either side (none in Proteus), the rest of the plate persisting as carti- 

 lage. The anterior end of the plate is often prolonged as an epipubic 

 process (fig. 280), and in lunged Salamandrina this is replaced by an 

 independent ypsiloid cartilage (fig. 281), divided at the tip, which 

 affords origin for respiratory muscles. Its homology with the 

 epipubis is not certain. It arises apart from the pubic cartilage and 

 develops later than the other pelvic cartilages. The ilium is always 

 short, is more or less ossified and is connected with a single sacral 

 vertebra by ligament or direct articulation, its upper end often 

 persisting as cartilage. 



Anura have a pelvis (fig. 282) differing greatly in form from that 

 of Urodeles. In the early stages the halves of the ischio-pubic 



