186 CASTORID A. 
dyle; whilst, m the existing Huropean 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 inshape. 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 mstructive portion of the lower jaw of 
the Trogontherium in the collection of Charles Lyell, Esq., 
Fig. 72. 
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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 
