U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



PEROGNATHUS, Maxim. 



Peroffnathtu, PRINCB MAXIM. Nova Acta Acad. C. L. C. XIX, i, 1839, 369. 

 Cricetodiput, PKALB, Mammalia and Birds, U. S. 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 hullae 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. Th 

 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 ear 

 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 

 DipodomySj 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 



