1 882.] The Reptiles of the American Eocetie. 983 



known from the Manti beds of Utah. None are known from the 

 Miocene formations east or west of the Rocky mountains, but 

 they are not rare in the marine Miocene of the Atlantic coast. 

 All the species belong to two genera, Plerodon Meyer, and Cro- 

 codilus Linn. One species of the former is found in the Wasatch 

 beds, with three or four species of Crocodilus. In the Bridger 

 beds I know of six species of the latter genus. 



It is a fact that the American genus Alligator is nowhere found 

 in the Tertiary formations of our continent. It is evident that it 

 is a specialized form of Crocodilus, which first appeared in Europe 

 in Tertiary times, and subsequently in this country. 



Crocodilus Linn. 



The Eocene species of true crocodiles differ much in size and 

 characters, ranging from the C. heterodon, which is not larger than 

 an Iguana, to the C. antiquus and C. clavis, which rival the existing 

 species of the East Indies. 



The species are divided into two sections, which are distin- 

 guished by the form of the frontal bone. In the one it is thin, 

 and has low lateral olfactory crests. Such species are as yet only 

 known from the Wasatch formation. They are the C. grypus 

 Cope and C. loheeleri Cope. The species of the second section 

 have massive frontal bones with strong lateral olfactory crests. 

 The C. heterodon of the Wasatch belongs here; also the C. elliottii 

 of the Bridger, and the C. clavis of the Washakie basin. The 

 frontal bones of several of the species are unknown. The species 

 may be also distinguished by the sculpture of their teeth, some 

 having the crowns grooved or channeled, and others having them 

 smooth or finely lined. Of the former kind are CsubuLatus Cope, 

 C. acer Cope and C. sulciferus Cope ; all the other species come- 

 under the second head. The C. squankensis Marsh, from the 

 Eocene of New Jersey has the enamel peculiarly rugose. A 

 peculiarity of the composition of the crowns of some of the 

 species has been noticed, on account of which I proposed a genus, 

 Thccachampsa. In this type the crown is composed of concentric 

 hollow cones, one within the other. I have not been able to 

 separate the crowns of the recent crocodiles into such bodies, and 

 they are generally too thin to display more than a very few such 

 layers, were they so separable. This character was first observed 

 in some species of the Atlantic Coast, e. g., C. antiquus Leidy, and 



