496 



MONOGRAPnS OP NOIlTn AMBUICAN RODBNTIA. 



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

 any hitherto adduced by other writers: — 



Geuns Perogiialhua. 



Ocoipnt nearly plane, i. c, the mastoidH not pro- 

 jecting noticeably backof tbo occipital bouc. 



Apices of polronals separated by the whole width 

 of tbo basisphcnuid. 



Pnrietuls perfectly pentagonal, with nearly equal 

 sides. 



luterpariotol elliptical, much broader than long, 

 embraced between narrow plates of occipital. 



Ear with a dist iuct upright lobo of the antitragus, 

 and generally also a lobe of the tragus. 



Sole nuked to the heel, at least along a central 

 stripe. 



Size of Mu» mueculm, or much larger. 



Qenaa CricetoJipm, 



Occipnt with a broad omarginatiou, i. t,, the 

 mastoids bulging decidedly buck of the occipital 

 bone. 



Apices of petrosals almost meeting beneath the 

 basispheuoid. 



Parietals imperfectly pentagonal, iniequilateral. 



Interparietal pentagonal, shiold-sbaped,embraced 

 between merit spurs of the occipital. 



Ear with no vestige of a lobe either of antitragus 

 or tragus. 



Solo entirely hairy on the posterior half. 



Very diminative ; less in size than Miu miuculua. 



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 Cricelodipus 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 

 flat, as in most Rodents, while Cricelodipus shows an emargination, much shal- 

 lower and comparatively much broader than in Dipoilomys indeed, but still 

 well-marked. These cranial peculiarities, substantiating a genus Cricelodipus 

 distinct from Perognalhus, do hot appear to have been noted before the 

 appearance of my "Review". They arc 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. Cies.-Loop. Carol, six, 1839, 369. (Type /'. /a»cio<u».)— Codes, 



Proo. Pbila. Acad. 1875, !f79. (Excludes Cricelodipiu.) 

 < Perogiiatliiu, LeComtk, Proc. Acad. Nat. Sci. Phila. 1863, ^ii. (Includes CricetoiiijiiM.)— Baird, Hamm. 



N. A. 1857, 416. (Includes CrieefodipiM.)— Alston, Proc. ZoOl. Soc. Lond. 1876, 88. (Inolndca 



Cricetodipug.) 

 =:tAbr<m!it,' Gray, Proc. Zool. Soc. Lond. 1868,202. (Type "A. lordi", sp. n. =P. nuniioolat.) 



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. B. Alston, epitt. ined, Londini, 25 A^oti. 1876.— In penning my original account of this gronp 

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

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

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



umiiiTnii— iMwiwi 



M 



