Family CASTORID^. 



Taking Castor, which is the only well-known form, as the type of the 

 family, the Castorida present many points of resemblance to the Sciurida, 

 of which group the Castorida were formerly regarded as constituting a sub- 

 family. Both Castor and Haplodon {z^Aplodontia Richardson) are evidently 

 quite nearly allied to the Sciurida:, and, together with Jnomalurus 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 Castorida 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 difiers in many 

 important particulars. It not only lacks the strongly-developed post-orbital 

 processes seen in all the genera of the Sciurida, 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 

 Sciuridee, the molars are truly rooted, with multiple fangs, short crowns, and 

 a tiiberculated 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 



'Drogontherium (Owen), EmmIot, Palaocattor, Steneofiber, Ckalioomyt, and Cattoroida. 



431 



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