NATURAL SCIENCES OF PHILADELPHIA. 279 



Genus Perognathus. Genus Cricetodipus. 



Occiput nearly plane, i, e. , the mas- Occiput with a broad emargination, 



toids not projecting noticeably back i.e., the mastoids bulging decidedly 



of the occipital. back of the occipital. 



Apices of petrosals separated by Apices of petrosals almost meeting 



the whole width of the basisphenoid. beneath the basisphenoid. 



Parietals perfectly pentagonal, Parietals imperfectly pentagonal, 



with nearly equal sides. inequilateral. 



Interparietal elliptical, much broad- Interparietal pentagonal, shield- 



er than long, embraced between nar- shaped, embraced between mere 



row plates of occipital. spurs of the occipital. 



Ear with a distinct upright lobe of Ear with no vestige of a lobe either 



the antitragus, and generally also a of antitragus or tragus, 

 lobe of the tragus. 



Soles naked to the heels, at least Soles entirely hairy on the pos- 



along a central stripe. terior half. 



Size of Mus musculus, or much Very diminutive; much less in 



larger. size than Mus musculus. 



The cranial characters above adduced, it may be observed, are 

 all co-ordinated with the single main feature of much greater de- 

 velopment of the mastoid in Cricetodipus than in Perognathus, the 

 state of the parts in the former being an evident approach to the 

 peculiarities of Dipodomijs itself. The difference in the shape of 

 the occiput is very striking when skulls of the two genera are 

 laid beside each other the part in Perognathus being quite flat, 

 as in most rodents, while Cricetodipus shows an emargination, 

 much broader and shallower than in Dipodomys indeed, but still 

 well marked. These cranial peculiarities, substantiating a genus 

 Cricetodipus distinct from Perognathus, do not appear to have 

 been noted before. They are correlated with the excellent and 

 readily appreciable external characters of the feet and ears already 

 presented by Prof. Baird. 



I. Genus PEROGNATHUS, Maxim. 



=Perognathus, Maxim., Nov. Act. Acad. Caes.-Leop. Carol. XIX. 1839, 369. 

 Type, P. fasciatus. 



< Perognathus, LeConte, Proc. Acad. Nat. Sci. Phila. 1853, 224; includes Criceto- 

 dipus. Baird, Mamui. N. A. 1857, 416 ; includes Cricetodipus. 



Having already indicated the generic characters of Perognathus, 

 I need only here give some further details respecting the skull 

 and teeth, following with an analysis of the species. I have 



