494 



MONOGUAPIIS OF NORTH AMERICAN RODENTIA. 



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mys require modification. Perognathus is a link between the two families, 

 just as, in its own family, Cricetodipus is between Perognathus on the one 

 hand, and the extraordinarily modified Dipodomys on the other. 



This brings us to the consideration of what, if any, <livision may be made 

 of the family Saccotnyidce into groups of more t'lau generic value. The genus 

 Dipodomys is so exaggerated in some of its peculiarities that, as it seems to 

 me, its characters may properly he set over against those whicli Perognathus 

 and Cricetodipus share together. The leading antitheses may be indicated in 

 the following manner: — 



Subfamily PehdgnatiiidinjE. 

 (Perognalhm aud Cricetodiptii).) 



Cervical vertebrte free (t). 



Molars rooted, 



Auterior molar with a lobe in addition to the 

 main prism. 



Skull half as wide as long, and two-lbirds as 

 high as wide. 



Mastoids moderately developed (for this family). 



Tympanic little inflated. 



Occipital plane not, or not much, omarginate. 



Petrosals, though approximate, not in tontact 

 with each other, but with basi-occipital throughout. 



Zygomatic plate of maxillary of ordinary Bodont 

 character. 



Farietals pentangular. 



Interparietal wider than long. 



No pit on inner side of lower jaw near the molars- 

 Hind limbs little if any longer than the fore, not 

 very obviously saltatorial. 



Inner hind digit well developed and low down. 



Soles naked or sparsely pilous. 



Pelage comparatively coarse and harsh. 



Subfamily Dm>DUMVlNJS 



(Dipodomys alone.) 



2d, 3<1, and 4th cervical vertebrie anchylosed. 



Molars rootless. 



Ant«rior molar a simple prism. 



SknU two-thirds as wide as long, and half as high 

 as wide. 



Mastoids extraordinarily developed (even for this 

 family). 



Tympanic completely bullous. 



Occipital plane deeply emarginate. 



Petrosals in mutual contact at their apices, and 

 Assured away from basi-occipital. 



Zygomatic plate of maxillary roofing much of tho 

 orbit. 



Farietals triangular. 



Interparietal longer than wide. 



A deep pit on inner side of lower Jaw near tho 

 molars. 



Hind limbs elongated, Jerboa-like, highly salta- 

 torial. 



Inner hind digit rudimentary and elevated. 



Soles densely hairy, like a rabbit's. 



Pelage comparatively soft (for this family).— 



Having never seen a specimen of Heteromys, I am unable to say how 

 nearly this genus may coincide with the characters of Perognathidina as here 

 established ; Mr. Alston* seems to consider it closely related to Perognathus ; 

 80 does Hr. Peters ;t but it is my impression that it will prove to constitute 



* See his characters of these genera, as quoted in foot-note on a foregoing (page 490.) 

 t Hr. W. Peters (loc. supra cit. p. 355), endorsing Waterhonse's and Baird's views of tho constitution 

 of the group, proposes to divide it as follows : — 



a. Dlpodomyina, with rootless molars, grooved upper iDoisots, aud no spines in the pelage. 



Dipodomys Gray = Maorocoliu Wagner. 



b. Heteromyina, with rooted molars. 



t Upper incisors grooved. 



Perognalhut Maxim, and the subgenera Abromyt Gray and Cricelodipm Peale. 

 1 1 Upper incisors broad and smooth in front. 



Jletcromya Desmarest and Saccomya Vr. Cuv. 

 In characterizing his new species of Hcleromyt, H. adipernu, Hr. Peters gives (loc. snpro oit, pp. 



