A NEW FOSSIL ALLIGATOR FROM THE HELL CREEK 

 BEDS OF I^IONTANA. 



By Charles W. Gilmore, 



Assistant Curator of Fossil Reptiles, U. S. National Museum. 



The specimen described below belongs to the vertebrate paleon- 

 tological collection of the American Museum of Natural History, 

 New York, and it is through the generosity of Dr. H. F. Osborn and 

 'Mr. Barnum Brown, of that institution, that I now have the privilege 

 of describing it. 



Tliis specimen represents a true though primitive member of the 

 AlUgatoridse, as is abundantly shown by the general proportion of 

 the skull, especially in the shortness and flatness of the broadly 

 rounded muzzle and the nonconstriction at the maxillo-premaxillary 

 union, the reception of the anterior teeth in pits of the upper jaw, 

 the lower teeth biting witliin the upper, and the divergent lateral 

 borders of the anterior ends of the palatines. 



In North America three genera, Diplocynodon, Alligator, and 

 Bottosaurus, have been included under the family Alligatoridse. 



From Diplocynodon the present form is to be separated at once 

 by the nonconstriction of the snout at the maxillo-premaxillary 

 suture and the uniform size of the premaxillary teeth. 



The abbreviated facial region, the posterior extension of the nasal 

 bones, and differences in the dentition distinguish it from Alligator. 



On account of the lack of homologous parts for comparison, the 

 separation of this specimen from Bottosaurus is somewhat more 

 diflicult, though their distinctness appears to be incUcated. 



In Leidy's description ^ of the type specimen of Bottosaurus liarlani 

 he says: "The fragment of the dental bone is about 15 inches in 

 length, and in this extent contains the remains of 11 alveoU, wliich, 

 perhaps, comprise the whole number except three or four." In the 

 ]\Iontana skull the total length of the dental series is 8 inches and 

 contains alveoli for 19 teeth. Still further Leidy says the alveoh 

 "appear to indicate a succession of teeth related to one another in 

 size nearly as in the Crocodile and Alligator." The succession of 



1 Smithsonian Contributions to Knowledge, vol. 14, 1865, p. 13. 



Proceedings U. S. National Museum, Vol. 41— No. 1860. 



297 



