THE CE.OCODILIA. 249 



The pelvis (Fig. 76, F) is not cunnectecl by bone -with tbe 

 vertebral coliunn. It consists of an ilitim (II. ), an ischium 

 [Is.], and a pubis {Pb.), uniting together to form an aceta- 

 bulum, while the pubis and ischium of each side meet in 

 the middle line. The ischium is a naiTOw and almost rod- 

 like bone, the pubis is somewhat broader, especially at its 

 symphysial end. 



The hind limb (Fig. 76, D) has substantially the same 

 structure as the fore limb, but is always smaller, and gene- 

 rally of much less size. 



The only other bony structure appertaining to Ichthyo- 

 saurus that need be noticed, is a circle of plates developed 

 in the sclerotic of the enormous eye, which is frequently 

 met with in a very perfect state of presei-vation. 



It is possible that the Ichthyosauria occur in the Trias ; 

 they abound in the Lias and in other rocks of Mesozoic 

 date, up to> and including, the Chalk. 



Some attain gigantic dimensions, and many species have 

 been founded by the diiferences in form and proportion of 

 the body and of the teeth; but no one form is sufficiently 

 different from the rest to justify its separation as a distinct 

 genus. They may be roughly grouped into such as have 

 relatively short snouts and short paddles, with four carpalia 

 (J. intermedius, communis, &c.) ; and such as have longer 

 snouts, long paddles, and three carpalia (I. longirostris, 

 tenuirostris, platyodonj . 



VI. The Ceocodilia. — Crocodiles, the highest living 

 Reptilia, are Lacertilian in form, with long tails and four 

 well-developed limbs, the anterior pair being the shorter, 

 and possessing five complete digits, while the hind feet are 

 four-toed. With a single exception, the living species have 

 nails on the three preaxial (radial and tibial) digits, so 

 that two digits are without nails on the forefoot, and one 

 on the hind-foot. The feet are webbed, but the degree to 

 which the web is developed varies greatly. The nostrils are 

 situated at the end of the long snout, and can be closed. 

 The tympianic membranes are exposed, but a cutaneous 



