Q 



16 PROCEEDINGS OF THE ACADEMY OF 



bone. The coronoid process is remarkably small, not nearly at- 

 taining the level of the condyle, at the root of which it appears as 

 a minute backward sloping prickle-like spur. The condyloid 

 ramus itself is small, compressed and oblique. The principal 

 feature of the bone is an immense flaring lamina which arises as a 

 verge upon the back part of the lower border of the body of the 

 bone, and expands obliquely outward and upward, with a peculiar 

 twist. This plate-like process is longer than the condyloid ramus 

 itself, and ends in an acute point, so that the back of the jaw 

 appears two-pronged. There is a deep pit between the alveolus 

 and the root of the coronoid plate. The mental foramen is con- 

 spicuous upon the outside of the jaw close to the incisors. The 

 small size and lowness of the jaw is seen in the peculiarly retreat- 

 ing chin of the species ; and it is probable, especially to judge 

 from the condition of the coronoid, that the biting power is com- 

 paratively slight. 



The vertebral formula of Dipodomys ordii is given by Baird as 

 7 cervical (with anchyloses of 2d, 3d, and 4th), 12 dorsal, 9 lumbar, 

 4 sacral, and 28 caudal, = 60 ; there is doubtless an individual 

 variability of several of the caudal segments. There are five 

 metacarpals and metatarsals, though the inner one of each is 

 reduced. There are perfect clavicles. The tibia and fibula are 

 united below. 



C. Dental Characters of Dipodomys. 



The dentition of Dipodomys is simple. Of the four molar teeth 

 above and below, the anterior one is a premolar, being preceded 

 by a deciduous tooth, which, however, is long persistent. This 

 one is rooted and with a more complicated crt>wn than the rest ; 

 the molars proper are rootless and perennial. In both jaws, the 

 set of the molars is very oblique ; in the upper, the anterior tooth 

 bends strongly backward, and the posterior one somewhat forward; 

 thus bringing their crowns in close apposition, though their roots 

 are divergent. It is the same in the lower jaw, though the greatest 

 obliquity there is in the strong forward set of the posterior tooth. 

 The teeth are all simple compressed prisms, broader in the trans- 

 verse than in the fore and aft direction. In the worn state, the 

 crowns of the two intermediate molars are simply elliptical ; that 

 of the anterior molar is rather a spherical triangle, with convex 

 posterior, and two concave anterior sides; the back molar is small 



