494 MONOGRAPHS OF 
mys require modification. 
NORTH AMERICAN RODENTIA. 
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, division may be made 
of the family Saccomyide 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 following manner :— 
Subfamily PEROGNATHIDINA. 
(Perognathus and Cricetodipus.) 
Cervical vertebra free (?). 
Molars rooted. 
Anterior molar with .a lobe in addition to the 
main prism. 
Skull half as wide as long, and two-thirds as 
high as wide. 
Mastoids moderately developed (for this family). 
Tympanic little inflated. 
Occipital plane not, or not much, emarginate. 
Petrosals, though approximate, not in contact 
with each other, but with basi-occipital throughout. 
Zygomatic plate of maxillary of ordinary Rodent 
character. 
Parietals pentangular. 
Interparietal wider than long. 
No pit on innerside 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. 
The leading antitheses may be indicated in 
Subfamily DirropoMy1n2a 
(Dipodomys alone.) 
2d, 3d, and 4th cervical vertebra anchylosed. 
Molars rootless. 
Anterior molar a simple prism. 
Skull two-thirds as wide as long, and half as high 
as wide. i 
Mastoids extraordinarily developed (even for this 
family). 
Tympanic completely bullous. 
Occipital plane deeply emarginate. 
Petrosals in mutual contact at their apices, and 
fissured away from basi-occipital. 
Zygomatic plate of maxillary roofing much of the 
orbit. 
Parietals triangular. 
Interparietal longer than wide. 
A deep pit on inner side of lower jaw near the 
molars. 
Hind limbs elongated, Jerboa-like, highly salta- 
torial. 
Inner hind digit rndimentary and elevated. 
Soles densely hairy, like a rabbit’s. 
Pelage comparatively soft (for this family). 
Having never seen a specimen of Heteromys, 1 am unable to say how 
nearly this genus may coincide with the characters of Perognathidine as here 
established; Mr. Alston* seems to consider it closely related to Perognathus ; 
SO 9 does Hr. Peters ; it but it is my impression that it will prove to constitute 
* See his iain of these genera, as quoted in foot-note on a foregoing (page 490.) 
| Hr. W. Peters (loc. supra cit. p. 355), endorsing Waterhouse’s and Baird’s views of the constitution 
of the group, proposes to divide it as follows ; — 
a. Dipodomyina, with rootless molars, grooved upper incisors, ‘and no spines in the pelage. 
Dipodomys Gray = Mnorocolna Wagner. 
b. Heteromyina, with rooted molars. 
t Upper incisors grooved. 
Perograthus Maxim, and the subgenera Abromys Gray and Cricetodipus Peale. 
tt Upper incisors broad and smooth in front. 
Heteromys Desmarest and Saccomys Fr. Cuy. 
In characterizing his new species of Heteromys, I. adspersus, Hr. Peters gives (loc. supra cit. pp. 
