494 MONOGRAPHS OF NORTH AMERICAN BODENTIA. 



mys require modification. Perognathus is ;i 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 limits us in the consideration of what, if an}', division may be made 

 of the family Saccomyidce into groups of more than generic value. The genus 

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

 me, its characters may properly be set over against those which Perognathus 

 and Cricetodipus share together The leading antitheses may he indicated in 

 the following manner: — 



Subfamily PeroGNAthidin^. Subfamily Uii'odomyinjB 



(Perognathus and Cricetodipus.') (Dipodomys alone.) 



Cervical vertebras free (?). 2d, 3d, and 4th cervical vertebno auebylosed. 



Molars rooted. Molars rootless. 



Anterior molar with a lobe in addition to the Anterior molar a simple prism. 

 main prism. 



Skull half as wide as long, and two-thirds as Skull two-thirds as wide as long, and half as bigh 



high as wide. as wide. 



Mastoids moderately developed (for this family). Mastoids extraordinarily developed (even for this 



family). 



Tympanic little inflated. Tympanic completely bullous. 



Occipital plane not, or not much, emarginate. Occipital plane deeply emarginate. 



Petrosals, though approximate, not in contact Petrosals in mutual contact at their apices, and 



with each other, but with basi-occipital throughout, fissured away from basi-occipital. 



Zygomatic plate of maxillary of ordinary Rodent Zygomatic plate of maxillary roofing much of the 



character. orbit. 



Parietals pentangular. Parietals triangular. 



Interparietal wider than long. Interparietal longer than wide. 



No pit on innersideof lower jaw near the molars. A deep pit on inner side of lower jaw near tho 



molars. 



Hind limbs little if any longer than the fore, not Hind limbs elongated, Jerboa-like, highly salta- 



vcry obviously saltatorial. torial. 



Inner hind digit well developed and low down. Inner hind digit rudimentary and elevated. 



Soles naked or sparsely pilous. Soles densely hairy, like a rabbit's. 



Pelage comparatively coarse and harsh. 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 Perognatliidince as here 

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

 so does Ilr. Peters;f but it is my impression that it will prove to constitute 



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

 t llr. W. Peters (loe. supra eit. p. 355), endorsing Waterhouse's and Baird's views of the constitution 

 of tin- group, proposes to divide it as follows : — 



a. Dipodomyina, with rootless molars, grooved upper incisors, and no spines in the pelage. 



Dipodomys Gray = Macroeolns Wagner. 



b. Beteromyina, with rooted molars. 



t Upper incisors grooved. 



Perogr.athus Maxim, and the subgenera Abromys Gray and Cricetodipus Peale. 

 1 1 Upper incisors broad and smooth in front. 

 Ucti-romjis Dcsmnrcst and Saccomys l'r. Cuv. 

 In characterizing his new species of Ihttrumys, If. adspereus, Hr. Peters gives (loc. supra cit. pp. 



