IH76.] MR. E. R. ALSTON ON TUE ORDER GLIRES. 07 



Castoridce 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 nume- 

 rous 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 Castoridce 

 is a very isolated and aberrant family ; but they do not appear to 

 indicate any specially murine affinities. 



Fie. 2. 



Mandible of C'ricetomys 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 Castoridce, and 

 especially that of the Miocene genus Stenofiber. It differs from 

 both these groups, however, in the possession of a large infra- 

 orbital opening, and should form, as it appears to me, a fifth family 

 of the Sciuromorpha, under the name of Ischyro/nyidcef. 



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, pi. ravi. : 

 Cope, Report U.S. Geol. Survey, 1873, p. 477. 



