Famiry CASTORIDZE. 
Taking Castor, which is the only weil-known form, as the type of the 
family, the Castoride present many points of resemblance to the Sciurida, 
of which group the Castoride were formerly regarded as constituting a sub- 
family. Both Castor and Haplodon (= Aplodontia Richardson) are evidently 
quite nearly allied to the Sciwride, and, together with Anomalurus and the 
extinct North American Ischyromys, are properly placed by Alston in his 
Sciuromorphic series, each being regarded as the type of a distinct family. 
Castor differs from any of the Squirrels in being adapted to an aquatic 
mode of life, and presumably several, but probably not all, of the extinct 
genera* commonly referred to the Castoride were similarly modified. While 
the skull in all the genera referred to the Beaver group presents in general 
form some similarity to that of the Squirrels, it constantly differs in many 
important particulars. It not only lacks the strongly-developed post-orbital 
processes seen in all the genera of the Sciwrid@, but the molar series are 
not widely separated, and converge anteriorly instead of being parallel, and 
the palatal area of the intermaxillaries is arched instead of plane. In the 
Sciuride, the molars are truly rooted, with multiple fangs, short crowns, and 
a tuberculated triturating surface, and undergo much change as a result of 
attrition. In Castor, the molars have a long-persistent dentinal pulp, the 
teeth continuing to grow for a long period, and becoming truly rooted only late 
in life; they are single-rooted, have no well-defined coronal portion, and the 
triturating surface consists of a complicated infolding of the enamel-border 
of the tooth, and undergoes little change by attrition; they decrease in size 
posteriorly. The lower jaw is more massive, and the rami are more firmly 
united by a much longer symphysial surface. The incisors are of very large 
*Trogontherium (Owen), Eucastor, Palewocastor, Steneofiber, Chalicomys, and Castoroides. 
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