378 Prof. II. G. Scelcy on the Dentition of the 



reputed incisors of Tritylodojk are canine teeth, the inci 

 being lost earlier than the middle incisors of Gomphognathm 

 polyphagia. Professor II. F. Osborn states, in the ' Ameri- 

 can Naturalist' for May 1898, that I figured a portion of the 

 lower jaw of Tritylodon ; but no generic determination was 

 made of that fossil. 



r J 'he specimen belongs to a larger animal than Tritylodon 

 longcevus. The intractable matrix which obscured the alveolar 

 bonier in the mandible has now been removed, and the jaw is 

 referable to a species or subgenus of Gomphognathm near to 

 G. polypliaguf*. 



This mandibular fragment is 2h inches long from the 

 incisor teeth to the first molar tooth. It is separated from 

 the missing left ramus by fracture, but the rami were united 

 by close bony union, and the socket of the first incisor of the 

 left ramus remains with this specimen. The crown of that 

 tooth may have perished during the life of the animal, though 

 the larger part of the root remains, shown in a vertical 

 fracture. The symphysial surface, about 1^ inch long, 1 inch 

 deep in front, and f inch deep posteriorly, was of long ovate 

 form. 



The inferior external surface of the jaw is convex from front 

 to back, and from side to side slightly convex in front but 

 somewhat flattened. This convex chin surface makes an angle 

 with the relatively vertical external lateral surface, which is 

 gently convex from above downward. The lateral surface is 

 | inch deep at the canine tooth and increases in depth as it 

 extends backward. The internal surface of the jaw above the 

 symphysis is a channel, nearly straight from front to back, 

 sunk well below the level of the canine and the anterior half 

 of the diastema. 



The three incisor teeth are close-set. They occupy a width 

 of half an inch. The crowns are broken, but they are nearly 

 uniform in size, nearly circular, with a slight transverse 

 natural compression. The third incisor is in front of the 

 canine. The second and first incisor teeth are further forward 

 successively, so as to make a curved external contour, much 

 in the manner of Gomphognathus kannevieyeri, which is the 

 only species with the mandible separated from the skull. 



The canine tooth is directed upward and forward, and not 

 outward as in G. kanntmeyeri, so that there is no appreciable 

 lateral bulbous expansion of the extremity of the mandible as 

 in that species. The tooth is strong, laterally compressed, 

 ovate in transverse section on the broken surface, -^ inch 

 from front to back at the fracture, and ^ inch wide, but 

 slightly wider anteriorly. What remains of the external 



