278 



PROCEEDINGS OF THE ACADEMY OF 



Subfamily Perognathidin^:. 



(Perognathus and Crketodipus.) 



Tympanic little inflated. 



Occipital plane not, or not much, 

 emarginate. 



Petrosals, though approximate, 

 not in contact with each other, but 

 with the basi-occipital throughout. 



Zygomatic plate of maxillary of 

 ordinary rodent character. 



Parietals pentangular. 



Interparietal wider than long. 



No pit on inner side of jaw near 

 the molars. 



Hind limbs little if any longer 

 than the fore, not very obviously 

 saltatorial. 



Soles naked of sparsely pilous. 



Pelage comparatively coarse and 

 harsh. 



Inner hind digit moderately de- 

 veloped and low down. 



Subfamily Dipodomyin^:. 



(Dipodomys alone.) 



Tympanic completely bullous. 

 Occipital plane deeply emarginate. 



Petrosals in mutual contact at their 

 apices, and fissured away from basi- 

 occipital. 



Zygomatic plate of maxillary roof- 

 ing much of the orbit. 



Parietals triangular. 



Interparietal longer than wide. 



A deep pit on inner side of jaw 

 near the molars. 



Hind limbs elongated, jerboa-like, 

 highly saltatorial. 



Soles densely hairy, like a rabbit's 

 foot. 

 Pelage comparatively soft. 



Inner hind digit rudimentary and 

 elevated. 



I am unable to say how nearly the genera Saccomys and Hetero- 

 mys may coincide with the characters of Perognathidinee as here 

 established ; but it is my impression that they will prove to con- 

 stitute a third subfamily. In such case, some of the characters 

 of the three would be as follows : 



Dipodomyin^e. Molars rootless ; upper incisors compressed, 

 sulcate. 



PEROGNATiiiDiNiE. Molars rooted ; upper incisors compressed, 

 sulcate. 



SACcoMYiNiE. Molars rooted ; upper incisors broad, smooth. 



Subfamily Perognathidinee, Coues. 



The characters of the group having been already sufficient^ 

 elucidated, it only remains to note the two genera by which it is 

 represented in North America. These may be readily distinguished 

 by much stronger characters than any hitherto adduced. 



