292 THE VERTEBRATE SKELETON. 



bearing a forwardly projecting obturator process. In Ptero- 

 sauria the ischium is fused with the ilium, and in both ptero- 

 saurs and crocodiles the ilium and ischium are the only bones 

 taking part in the formation of the acetabulum. In most 

 Lacertilia there is an unpaired structure,, the hypo-iachium or 

 os cloacae projecting back from the symphysis ischii, which is 

 usually separated from the symphysis pubis by a large space, 

 the foramen cordiforme. In some Lacertilia and Chelonia 

 there is a cartilaginous bar dividing the foramen cordiforme 

 into two obturator foramina ; in many Chelonia this bar is 

 ossified. Among Ophidia, Python, Tortrix, Typhlops and their 

 allies have a structure representing a vestigial ischio-pubis : 

 but in most Ophidia there is no trace of the pelvis. In some 

 Theromorpha all the bones of the pelvis are completely fused, 

 forming an os innominatum as in mammals ; the pubes and 

 ischia are so completely fused that sometimes as in Paria- 

 saurus even the obturator foramina are closed. 



Concerning the reptilian pubis there are considerable diffi- 

 culties. Sometimes there is only a single pubic structure 

 present, sometimes there are two. The reptilian pubis is best 

 understood by comparing the arrangements met with in the 

 various other groups with that in the Orthopod Dinosaurs 

 such as Iguanodon. In Iguanodon the pubis consists of two 

 portions, viz. of a moderately broad pre-pubis directed down- 

 wards and forwards, and of a narrow greatly elongated post- 

 pubis directed backwards parallel to the ischium. The pubis 

 is united to both ilium and ischium, the acetabulum has a 

 large unossified space, and neither pre-pubes nor post-pubes 

 meet in ventral symphyses. The arrangement bears a great 

 resemblance to that of Ratite birds. In Lacertilia, Chelonia, 

 Rhynchocephalia and Ichthyosauria together with Theropod 

 and Sauropod Dinosaurs the pubis corresponds to the pre-pubis 

 of Iguanodon and is a more or less cylindrical bone expanded 

 at both ends, meeting its fellow in a ventral symphysis. In 

 Chelonia and Lacertilia the pubis bears a lateral process which 



