REPTILIA. 385 



is a rudimentary ilium. Other Reptilia have a pelvis composed of ilium, 

 pubis and ischium. The iliac axis slopes forwards and downwards, and the 

 bone usually extends backwards, i. e. behind its point of union with the 

 sacral ribs, but in the Crocodilia, and especially in many Deinosauria, e. g. 

 Iguanodon, Stegosaurus, it has a forward extension indicated in many Lacer- 

 tilia by a process (processus acetabularis). The acetabulum is formed by the 

 three bones jointly, except in the Crocodilia where the pubis is excluded 

 from it. The centre of this cavity is fibrous in Crocodilia and many 

 Deinosauria. As a rule, there is a pubic and ischial symphysis, but the 

 former is wanting in many Deinosauria, e. g. Iguanodon. The pubis varies 

 in shape and size. In the Stegosauria and Ornithopoda among Deinosauria 

 it has a peculiar shape ; one portion, the post-pubis, being long and slender 

 and parallel to the ischium which, in these two groups, is strongly inclined 

 backwards as in Aves ; the other, the praepubis, projecting downwards and 

 forwards. The latter is probably the homologue of the pectineal tubercle. 

 There is no evidence that the two portions represent two distinct bones 

 (see p. 65). There is a large obturator foramen between the pubis and 

 ischium of the same side, but the obturator nerve either perforates the 

 pubis itself (Lacertilia] or the fibrous membrane closing the foramen. In 

 land and fresh-water Chelonia, the pubes and ischia touch one another in 

 the median line ; but in the marine Chelonia, in the Crocodilia and Lacer- 

 tilia, the two obturator foramina are only separated medianly by a strong 

 fibrous band extending from the former to the latter. There is a small 

 epipubic cartilage in many Lacertilia and Chelonia, and in many of the 

 former an os cloacae extends backwards from the ischial symphysis below 

 the cloaca. 



The Ophidia with a few exceptions (supra) are limbless. Certain 

 Lacertilia (most Amphisbaenoidea, some Brevilingtiia, e.g. Anguis] are in the 

 same condition. Others possess rudimentary limbs with or without a 

 reduced number of toes. In other Reptilia limbs are well developed. They 

 are short and paddle-like in Plesiosauria and Ichthyosauria, and in the 

 latter the digits often bifurcate, and there are rows of extra ossicles at their 

 margins. The Ornithopoda among Deinosauria appear to have walked erect, 

 and their fore-limbs are short. The Pterodactyles have the fifth finger 

 elongated to support the wing membrane, and the fore-limb is large and 

 powerful in them. But as a rule, the limbs are small in comparison with the 

 body. There is much variety in the conformation of the carpus and tarsus, 

 of the digits and their phalanges, but the following points may be noted. 

 The carpus of some Chelonia possesses all the typical ossicles separate. In 

 the tarsus the ankle-joint comes to lie between the proximal and distal 

 tarsals in many Chelonia, the Lacertilia, the Ornithopoda among Deino- 

 sauria : and in the Ornithopoda the astragalus has an ascending process 

 more or less developed and at least applied to, or in some,instances perhaps 



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