114 



COMPARATIVE ANATOMY 



anterior epipubic process is paired throughout life (Fig. 90, A, B). 

 In the Derotremata and Myctodera, on the other hand, the 

 epipubis is unpaired from the first, owing probably to an abbrevia- 

 tion of development, its anterior end becoming bifurcated 

 secondarily (Fig. 90, C, D). 



As in Fishes and Dipnoans, the two halves of the ischio-pubic 

 region tend to fuse together in the middle line to form an un- 

 paired pelvic plate, but all kinds of modifications 

 occur in this respect in adaptation to the move- 

 ments of the hind-limb in different forms ; and, 

 as in all cases the median zone of the plate 

 represents the line of least resistance, the lateral 

 halves may eventually become more or less dis- 

 tinct from one another. The effect of the action 

 of the muscles becomes, however, greater when 

 the pubic region is more distinctly marked off 

 from the ischium, and ossification takes place 

 in it (e.g., Salamandra atra and, more rarely, S. 

 maculata). Thus the typical triradiate arrange- 

 ment of the pelvis (ilium, iscJmim, and pulis}, 

 such as is further differentiated in certain Stego- 

 cephala (Discosaurus) and in Reptiles, as well 

 as in Xenopus, is already sketched out (Fig. 91). 



An important difference between the pelvis 

 of Ganoids and Dipnoans and that of Amphibians 

 is seen in the marked development of the iliac 

 region in the latter group. The ilium, like the 

 scapula, extends upwards in the lateral walls of 

 the body (compare the iliac process of Elasmo- 

 branchs, Fig. 88), and in Proteus and Amphiuma, 

 owing to the reduction of the limbs in these 

 forms, does not reach the vertebral column (Fig. 

 90, A, B). In all other Amphibia, as in the 

 Amniota, it comes into connection with the 

 sacrum (p. 45), owing to the necessity for the 

 hind-limb to act as a support for the body in 

 terrestrial animals, and not merely as an organ 

 of propulsion, as in Fishes. 



Anura. The pelvis of the Anura differs from 



that of Urodela in the following characteristics. In correspondence 

 with their mode of progression, the ilium of each side becomes 

 extended so as to form a long rod (Figs. 91 C, 92) ; and the flat 

 pelvic plate, which in Urodeles lies in the plane of the abdominal 

 walls, becomes closely pressed together in the middle line and gives 

 rise to a well-marked ventral keel : it is not perforated by the 

 obturator nerve. The pubic region, moreover, though often calcified, 

 is independently ossified only in the case of Xenopus (Fig. 91, A,B). 



Reptiles.' The chief characteristics of the Reptilian pelvis as 



FIG. 92. PELVIC 

 ARCH OF FROG 



{ Ra na esculent a}. 

 From below. 



J, J 1 , ilium ; Is, is- 

 chium ; P, carti- 

 laginous pubic 

 region ; Or, the 

 median ventral 

 ischio-pubic 

 crest ; G, aceta- 

 bulum ; Oc, uro- 

 style ; Pt, trans- 

 verse process of 

 sacral vertebra. 



