SKELETON. 



171 



tion is well developed ; and when bones are developed in the car- 

 tilage, three elements can be recognized in each half, — a dorsal 

 ilium, and, below, an anterior pubis and a posterior ischium, the 

 obturator foramen either forming a part of the opening between 

 these two bones, or passing through the pubis itself. Ventrally 

 these bones can unite with their fellows of the opposite side in 

 a symphysis, while all three of a side meet in the acetabulum.* 

 These parts can be well compared with those of the pectoral 

 girdle ; pubis with procoracoid, ischium with coracoid, and ilium 

 with scapula ; but one difference 

 is to ' be noted, — the ilium be- 

 comes connected with the sacral 

 vertebra or vertebrae by the inter- 

 vention of short ribs (p. 146). To 

 these parts in the amphibia there 

 is frequently added in front of 

 the pubis a cartilaginous epipubis. 

 This reappears again in certain 

 reptiles ; and in mammals it may 

 be homologous with the so-called marsupial bones, which project 

 forward from the anterior margin of the pubis, there being two 

 views upon this point. These parts may undergo many modi- 



FiG. 183. Side view of pelvis of 

 opossum, after Minot. Ac^ acetabu- 

 lum ; y, obturator foramen ; /Z, ilium ; 

 JS, ischium; M^ marsupial bone. 



Fig. 184. Modifications of branchial arch and rays according to Gegenbaur's 

 archipterygium theory. 



fications, but the pelvis is not re-enforced by membrane bones 

 such as play such a part in the shoulder girdle. 



1 In many mammals a distinct acetabular bone occurs at the junction of the three. 



