282 ZOOLOGY. 



system of the United States ; one, Geosauris mitchelU, from New Jersey, 

 the other, Mosasaurus maxiinilia7ii, from the banks of the Yellowstone 

 River. Dicynodon is a highly anomalous genus found recently in South 

 Africa ; and, in some peculiarities, exhibiting a resemblance to the Chelonia 

 and to birds. There are no teeth at all along the margins of the jaws ;. but 

 below each orbit there is a single cylindrical acute tooth, which appears to have 

 groAvn at the base as it Avas worn away at the apex (as in the Rodentia), and 

 presenting a character entirely unique among reptiles. Other genera are 

 PalcLosaums^ Thecodontosmirus^ RliyncJiosaurus, &c. There are obscure 

 indications of species of ViD'anus, Iguana, Lacerta, iSclncus, &c., or of very 

 closely allied genera. 



We come now to the consideration of Saurians, of a type of organization 

 in many respects much superior to those hitherto considered. The most 

 striking feature consists in the rhizodont character of the teeth, or the 

 implantation of the teeth either in so many distinct sockets, or else in a 

 deep furrow. Some authors make a distinct order of Rliizodontes. Com- 

 paratively few of the species known belong to the fauna of the present 

 day ; the remainder being entirely extinct, and exhibiting forms the most 

 different from those which now abound on the surface of the earth. But 

 one family has living representatives, and most of its species are now 

 extinct. 



This family, CrocodilidcE, the eighth of our classification, is readily 

 distinguishable from other recent fiimilies by important characters, in 

 addition to the implantation of the teeth in distinct sockets. The tongue is 

 broad, fleshy, and entirely attached between the rami of the lower jaw. The 

 latter are longer than the cranium. The skin of the head is closely 

 adherent to the subjacent bone, and is without scales. The external nostrils 

 are close together at the superior extremity of the snout, and are provided 

 .with movable valves. The posterior nostrils open in the pharynx, and not 

 in the mouth, as in other saurians. The external ears also present an 

 unique character, in being provided with two movable opercula, simulating 

 the concha of the mammalian ear. The pupil consists of a vertical slit. The 

 skin of the back is covered with solid bony shields, with longitudinal 

 carina?, and arranged in longitudinal bands. The belly is covered with trans- 

 verse series of square plates. The greatly compressed tail is surmounted 

 by longitudinal crests, and provided with whorled square plates. The anterior 

 extremities have five distinct toes, the tAvo exterior Avithout claws ; the pos- 

 terior have but four toes, Avhich are palmated, or semi-palmated ; three 

 only have claAA\s. The penis of the male is simple, and the cloaca opens by a 

 longitudinal slit, instead of the double penis and transverse slit of most other 

 saurians. 



The living CrocodiUdm are most abundant in the Noav World, Avhere 

 about one half of the entire number is found. Europe presents us Avith not 

 a single species, Asia has three, and Africa perhaps four. Among the 

 crocodiles of the present day, Ave distinguish three distinct genera, the 

 alligators or caymans {Alligator), the true crocodiles {Crocodilus), and the 

 gavials (Gavialis). The tAA-o first genera are distinguished from the last 

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