274 PROCEEDINGS OP THE ACADEMY OP 



other. My impression is, however, that characters of more than 

 generic value will be found to distinguish them. 



However the case may stand respecting Heteromys and Saccomys, 

 concerning which precise information is greatly to be desired, it 

 is demonstrable that the genus Dipodomys stands quite isolated 

 from Perognathus and Cricetodipus by its extraordinary cranial 

 peculiarities. This circumstance throws the North American 

 o-enera into two distinct subfamilies, very easy to characterize. 



In these preliminary remarks I wish, furthermore, to insist upon 

 the full generic distinctions between Perognathus and Criceto- 

 dipus. The latter was first satisfactorily characterized as a sub- 

 genus by Prof. Baird, upon certain obvious and eligible external 

 characters alone. But examination of the skull, which Prof. 

 Baird, it seems, did not make, has satisfied me that the cranial 

 characters are fully up to the current generic mark. Cricetodijnis, 

 in fact, makes a decided step away from Perognathus in the 

 direction of Dipodomys, though still falling far short of the exag- 

 gerated peculiarities of the latter. 



Family SACCOMYID^E (Baird). 



= Saccomyinm, Baird, Maram. N. A. 1857, p. 404. (Subfamily.) 



= Saccomyince, Gray, P. Z. S. 1868, 199. (Family.) 



= SaccomyidcB, Gill, Arrang. Fam. Mamni. 1872, p. 21, No. 100. 



Chars. 1 Skull light, thin, and papery, with few if any decided 

 angles or ridges; rostrum elongate, attenuate, and tapering ; nasals 

 projected beyond incisors. Inter-mastoid width not less than 

 the inter-zygomatic, sometimes much greater. Inter-orbital space 

 much wider than the rostrum. Occipital region formed largely or 

 mostly of the mastoids. Palatal surface nearly flat and horizon- 

 tal. No anteorbital foramen in a usual site, but a large, rounded 

 perforation of the side of the maxillary, instead. Zygomata 

 slender, depressed in position, almost or quite abutting behind 

 against the tympanic ; malar thread-like. A delicate scroll-like 

 lachrymal, easily detached. Frontal broadly trapezoidal. Parietal 

 broad, triangular or pentagonal. A large interparietal, embraced 



1 The characters are drawn from Perognathus, Cricetodipus, and Dipo- 

 domys; but there is every reason to believe that no essential modification 

 of the phrase is required to embrace Saccomys and Heteromys as well. 

 Special reference is had to antithesis with Qeomyida. 



