214 THE ANATOMT OF VERTEBRATED ANIMALS. 



It is possible that the Ichthyosauria occur in the Trias" 

 tbej 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 differences 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 {I. inter- 

 medius, communis, etc.) ; and such as have longer snouts, long 

 paddles, and three carpalia {I. longirostris, tenuirostris, pla- 

 tyodo7i). 



VI. The Ceocodilia. — Crocodiles, the highest living -Rep- 

 tilia, "are Lacertilian in form, with long tails and four well-de- 

 veloped limbs, the anterior pair being the shorter, and pos- 

 sessing 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 fore-foot, 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 tympanic membranes are 

 exposed, but a cutaneous valve, or earlid, lies above each, and 

 can be shut down over it. All are partially aquatic in habit, 

 and some (the Gavials) are completely so. None of the exist- 

 ing genera are marine, though many ancient Crocodilia inhab- 

 ited the sea. 



The dermal armor is composed of scutes covered by epi- 

 dermic scales of corresponding form. When the armor is 

 complete — as in Oaiman and Jacare alone among existing 

 Crocodilia, in Teleosaurus and Stagonolepis among extinct 

 forms — it consists of transverse rows of quadrate bony plates, 

 disposed so as to form a distinct dorsal and ventral shield, 

 separated by soft integument, in the trunk, but united into 

 continuous rings on the taU. The scutes of the same row are 

 united suturally ; those of each row overlap their successors, 

 which present smooth facets to receive their under-surfaces. 

 In existing Crocodilia, in the extinct Crocodilus SastingsioB, 

 and in Stagonolepis, each ventral scute consists of two pieces, 

 a small anterior and a large posterior, united by a suture, 

 The scutes always exhibit a pitted sculpture, and those of the 

 dorsal region are ridged longitudinally, while the ventral 

 scales are always flat. More or fewer dorsal scutes exist it) 



