264 



VERTEBRATE SKELETON 



median ventral side, partially separating pubic and ischiadic parts. 

 This gap persists in the adult, and when the halves meet in the 

 ventral symphysis, the gaps become ischio-pubic fenestrae. usually 

 separated by hgament, rarely (Chelonia) by cartilage or bone in the 

 middle line from ischium to pubis. This median cartilage may 

 extend through the symphysis and in front of the pubes as an 

 epipubis (epigastroid), behind the ischia as a hypoischium (h)^o- 

 gastroid). These median parts may remain cartilage or may ossify 

 as independent bones, the hypoischium sometimes being called an OS 

 cloacae when it becomes involved in the cloacal wall. 



The term os cloacae is also given to paired bones in the wall of the cloaca of 

 a few lizards; nothing is known of their history but they are evidently not the 

 same as the hypoischium (fig. 284). 



Fig. 283. — Pelvis of 23 cm. Sphenodon (Schau- 

 insland, '03). e, epipubis; /li; hypoischium; il, 

 ilium; is, ischium; /, pubo-ischiadic ligament; o, 

 obturator foramen; p, pubis. 



Fig. 284. — Os cloacae of Gonatodcs 

 (Noble, '21). 



Many fossil reptiles lack the ischio-pubic fenestra, pubis and 

 ischium of the same side meeting in a continuous Hne. In others, as 

 in Stegocephals, the ossifications are small, the cartilages apparently 

 having been much larger, and in these the presence or absence of 

 fenestrae cannot be decided. In some groups (Squamata, Rhyncho- 

 cephala) there is a separate obturator foramen in each pubis, but in 

 most modern orders the obturator nerve passes through the fenestra 

 as in mammals. Which condition is the more primitive is uncertain. 

 Amphibia throw no Ught on the matter as the living species have no 

 ischio-pubic fenestra. 



Except in Dinosaurs, the ilium Is smaller, as a rule, than the 

 other pelvic bones, and is usually connected with two sacral verte- 

 brae, the exceptions among living species being snakes, and some 

 apodal lizards in which the pelvis is reduced or absent, while in Ichthy- 

 osaurs and some Pythonomorphs the reduced pelvis and vertebral 



