416 U. Ss. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
PEROGNATHUS, Maxim. 
Perognathus, Prince Maxim. Nova Acta Acad. C. L. C. XIX, 1, 1839, 369. 
Cricetodipus, Peatx, Mammalia and Birds, U. 8. Ex. Ex. 1848, 52. 
Incisors with a longitudinal groove down their anterior surface. Molars rooted; tuberculated in the young, in the adult 
with the crowns plane. The auditory bullae are much developed, and project behind slightly beyond the plane of the 
occipital bone; the zygoma is slender and low. ; 
The thumb and great toe are quite rudimentary ; the former with a rounded nail, the latter with a short claw. The 
remaining four digits are nearly equal; the claws on the fore feet longer than behind. The soles and palms are sometimes 
naked, sometimes hairy posteriorly. The tail is as long as the body, and covered with short hair; thinly so in the young. 
The external cheek pouches open amply, the slit extending from the angle of the mouth nearly to the scapula. * The ears 
are small ; in some species the antitragus is developed as a small rounded projecting lobe. 
The skull of Perognathus agrees in many respects very closely with that of Dipodomys. 
Thus, there is the same elongated muzzle and its semi-tubular projection beyond the incisors ; 
a similar development of the temporal bone; the thread-like malar; a similar palate, incisive 
foramina, aperture in the side of the muzzle, grooved upper incisors, and their curvature and 
pressing outward of their walls, &c. There still remain important points of distinction, how- 
ever, by which the two may readily be distinguished. : 
The skull is considerably depressed and flattened above; its greatest height more than two 
thirds its width, and its width between the temporal bones just half the length. The nasal . 
bones are elongated; linear behind and in connexion with the upper edges of the processes from 
the intermaxillary, project beyond the incisors, as a short tube, open below. This tube, how- 
ever, is shorter and more open than in Dipodomys. The parietal bones are five-sided. The 
interparietal is also somewhat five-sided and much developed ; much broader than long, and 
largely in contact anteriorly with the parietals, but embraced laterally by narrow branches from 
the occipital, which pass forward and connect by an acute point with the parietals. In Dipo- 
domys this interparietal is very small, narrow, and longitudinal, in contact with a very small 
portion of the parietal, which is more or less three-sided. 
The temporal bone, though highly developed, is smaller than in Dipodomys. While it enters 
largely into the occipital surface (which is quite plane) it is confined to the sides of the occiput, 
the occipital having more prominence than in Dipodomys, and coming into the plane 
of the back of the head. The mastoid portion is less developed in front, the meatus auditorius 
being tubular anteriorly, with a projecting rim, instead of being a mere hole in the middle of 
a bone, without any raised margin. The petrous bones come far forward, but scarcely overlap 
the sphenoid and abut as in Dipodomys. The linear opening between the petrous bone and the 
base occipital in Dipodomys is closed. 
The upper face of the zygomatic process of the maxillary is not produced backward as a thin 
plate over the orbital cavity, but is much as in ordinary rodents. There is no ante-orbital 
foramen in the base of the zygomatic process or a little anterior to it; this being, as in 
Dipodomys, replaced by a large rounded aperature in the side wall of the snout, opening 
directly into the nasal cavity. 
The lower jaw is shaped much like that of Dipodomys, with a similar outward twist of the 
upper end of the postero-inferior angle. The coronoid process is also outside of the plane of 
the condyloid. The tubercle indicating the posterior extremity of the incisor, instead of 
