989 ZOOLOGY. 
system of the United States; one, Geosauris mitchelli, from New Jersey, 
the other, Mosasaurus mazximiliani, 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 
grown at the base as it was worn away at the apex (as in the Rodentia), and 
presenting a character entirely unique among reptiles. Other genera are 
Paleosaurus, Thecodontosaurus, Rhynchosaurus, &c. There are obscure 
indications of species of Varanus, Iguana, Lacerta, Scincus, &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 Rhizodontes. Com- 
paratively few of the species known belong to the fauna of the present 
day; the remainder being entirely extinct, and exhibitmg 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, Crocodilide, the eighth of our classification, is readily 
distinguishable from other recent families 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 longitudmal 
carinz, 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 two exterior without claws; the pos- 
terior have but four toes, which are palmated, or semi-palmated; three 
only have claws. 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 
saurlans. 
The living Crocodilida are most abundant in the New World, where 
about one half of the entire number is found. Europe presents us with not 
a single species, Asia has three, and Africa perhaps four. Among the 
crocodiles of the present day, we distinguish three distinct genera, the 
alligators or caymans (Alligator), the true crocodiles (Crocodilus), and the 
gavials (Gavialis). The two first genera are distinguished from the last 
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