PELVIC ARCH 151 



In Snake-like Lizards the pelvis undergoes degeneration, and 

 in Amphisbsenians vestiges merely of the ilium and pubis are 

 present ; in certain Snakes vestiges of a pubis alone occur. 



While the pelvis of Lizards and Chelonians show a certain 

 amount of similarity, that of Crocodiles exhibits special character- 

 istics and is of particular interest, as in some points it resembles 

 that of certain extinct forms. The pubes, which have at first a 

 transverse position, become later directed forwards much more 

 markedly than in Chelonians and Lizards, and thus the ischio- 

 pubic foramina (in which the obturator foramina are included) are 

 very wide, and are separated from one another by a fibrous cord 



FIG. 111. PELVIS OF Lacerta vivipara. From the ventral side. 



Ac, acetabulum, in which the three pelvic bones come together ; Cep, epipubis, 

 composed of calcified cartilage ; Fo l , obturator foramen ; Hpls, hypoischium, 

 which becomes segmented off from the hinder ends of the ischia in the 

 embryo as a paired structure ; /, ilium, with its small preacetabular process 

 tt, much more strongly developed in Crocodiles, Dinosaurians and Birds ; 

 Is, Ischium, forming a symphysis at Sis ; Lg, fibrous ligament ; P, pubis ; 

 PP, prepubis. 



(Fig. 112). All three elements chondrify independently, and then 

 unite in the acetabulum, which is perforated. The pubic bone 

 becomes shut out from the acetabulum by a cartilaginous imrs 

 acetdbularis, not represented in lower Vertebrates, formed from the 

 acetabular process of the ilium. The epipubis is contained in a 

 cartilaginous apophysis at the anterior (distal) end of the pubis, 

 which does not, like the ischium, unite with its fellow in a 

 symphysis : there is no hypoischium. 



The ilium becomes greatly broadened out in the antero- 

 posterior direction dorsal ly, where it is attached to the sacrum ; a 

 similar extension of the ilium occurs still more markedly in the 

 Theromorpha, Dinosauria, and Birds (Fig. 113), in which it 



