186 CASTORID.E. 



dyle; whilst, in the existing European Beaver, the dia- 

 stema between the summit of the incisor and the first molar 

 is little more than one-third of the extent from the incisor 

 to the articular condyle. The lower incisor of the Trogon- 

 therium not only differs in both absolute and relative size, 

 but also in shape. The anterior, or outer enamelled part of 

 the tooth, is more convex, and, in a transverse section of 

 the tooth (fig. 73, a), describes a semicircle. The inner, or 

 mesial surface of the incisor, which in the existing and 

 fossil Beavers is flat, is concave in the Trogontherium, as 

 is also the outer surface of the tooth. 



A well-marked sub-generic distinction, viz. the dispro- 

 portionate size of the anterior molar, is well shewn in the 

 figure above-cited, but I shall more particularly notice it in 

 connection with the instructive portion of the lower jaw of 

 the Trogontherium in the collection of Charles Lyell, Esq., 



Fig. 72. 



Trogontherium Cuvieri, nat. size. Fresh- water beds, Cromer. 



by whose kind permission the specimen is here figured and 

 described (figs. 72 and 73). It was discovered by Mr. Lyell 

 in the blue clay which rests upon the Norwich crag at 



