49G MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



ica. These may be readily distinguished by much stronger characters than 

 any hitherto adduced by oilier writers: — 



Genus Perognathus. Genus Cricetodipus. 



Occiput nearly plane, t. e., the mastoids not pro- Occiput with a broad emargination, i.e., the 



jecting noticeably back of the occipital bone. mastoids bulging decidedly bock of the ocoipital 



bone. 



Apices ofpetrosals separated by the whole width Apices of petrosals almosi meeting beneath the 



of the basisphenoid. basisphenoid. 



Parietals perfectly pentagonal, with nearly equal Parietals imperfectly pentagonal, inequilateral, 

 sides. 



Interparietal elliptical, much broader tliau long, Interparietal pentagonal, shirld-shaped,ernbraced 



embraced between narrow plates of occipital. between mere spurs of the occipital. 



Ear with a distinct upright lobe of the antitragns, Ear with no vestigo of a lobe either of antitragus 



and generally also a lobe of the tragus. or tragus. 



Solo naked to the heel, at least along a central Sole entirely hairy on the posterior half. 

 stripe. 



Size of Mus musculus, or much larger. Very diminutive ; less in size than AIus musouJits. 



The cranial characters above adduced, it may be observed, are all coor- 

 dinated with the single main feature of much greater development of the 

 mastoid in Cricetodipus than in Perognathus, the state of the parts in the 

 former being an evident approach to the peculiarities of Dipodomys 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 

 Hat, as in most Rodents, while Cricetodipus shows an emargination, much shal- 

 lower and comparatively much broader 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 the 

 appearance of my "Review". They are correlated with the excellent and 

 readily appreciable external characters of the feet and ears presented by 

 Professor Baird. 



Genus PEROGNATHUS, Maxim. 



= Perognathus, Maxim., Nov. Act. Acad. Ca;s.-Leop. Carol, xix, 1839, 369. (Type P. fasciatus.) — Coues, 



Proc. Phila. Acad. 1875,279. (Excludes Cricetodipus.) 

 < Perognathus, Lk.Conte, Proc. Acad. Nat. Sci. Phila. 1853, 224. (Includes Cricetodipus.) — Baikd, Mamm. 



N. A. 1857, 4 Hi. (Includes Cricetodipus.)— Alston, Proc. Zool. Soc.Lond. 1876,88. (Includes 



Cricetodipus.) 

 = TAbromye,* Gray, Proc. Zool. Soc. Loud. 1868,202. (Type "A. lordi", sp. u. =P. monticolat.) 



Having already indicated the generic characters of Perognathus, I need 

 only here give some further details respecting the skull and teeth, following with 



* Auct. E. R. Alston, epist. incd. Londini, 25 Aot'. 1876. — In penning my original account of this group 

 for Proc. Phila. Acad., I was at a loss to know what to do with Abromys ; so I simply copied Gray's notice 

 into my text, stating that I could not make it out at all, though I failed to see any difference between 

 "Abroinys " and Perognathus, and suspected "A. lordi " to be P. monticola. In this impression, it seems, I 



