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Show 1876.] MR. E. R. ALSTON ON THE ORDER GLIRES. 67 Castoridee among the Myomorpha, but on the boundary between them and the Sciuromorpha, remarking that the fibula is stout, and remains long separate from the tibia*. But the characters of these bones seem to me to be strictly sciurine ; for though they are more or less firmly attached to one another in aged individuals, yet they always appear to remain essentially distinct throughout their length. Less weight is now generally given to external characters than was the case when Prof. Brandt wrote ; and the purely adaptive differentiation of the teeth, feet, and tail cannot be allowed to outweigh the numerous and important characters which are at once evident on a careful comparison of the skulls and skeletons of a Beaver and a Marmot. These external peculiarities, coupled with those of the digestive, excretory, and generative organs, certainly show that the Castoridee is a very isolated and aberrant family; but they do not appear to indicate any specially murine affinities. Fig. 2. Mandible of Cricetomys gambianus. An interesting confirmation of these views as to the position of the Beaver is afforded by the fossil rodent of the American Miocene to which Professor Leidy has given the name of Ischyromys. In this form the dentition of the typical Sciuridce is combined with a form of skull which very closely resembles that of the Castoridee, and especially that of the Miocene genus Stenofiber. It differs 'from both these groups, however, in the possession of a large infraorbital opening, and should form, as it appears to me, a fifth family of the Sciuromorpha, under the name of Jschyromyideef. The second section, Myomorpha, is at once separated from either of the others by the single character of the complete fusion in the adult of the lower part of the tibia and fibula. Externally, the muffle and upper lip are as in the last section; and the tail is cylin- * Op. cit. pp. 7, 35. t Cf. Leidy, Journ. Acad. Philadelphia, 2nd ser. vol. vii. pp. 335-338 ol rev* • Cope, Report U.S. Geol. Survey, 1873, p. 477. '' ' ' 5* |
