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



outwards to an obtuse point. The condyloid process is much compressed and quite deep, with 

 upper and lower edges nearly parallel, an^l ends in an elongated ovate condyle. There is a 

 distinct tubercle at the outer base of the condyloid process in the gap between this and the 

 ascending point of the angle of the lower jaw. This covers the posterior extremity of the 

 incisor. The coronoid process is small and prickle-shaped. Its plane is a little exterior to 

 that of the condyloid process, and in a line with the tubercle just mentioned. There is a deep 

 pit between the last molar and the condyloid process, on its inner side. 



The palatine surface of the skull is very limited, although it all lies in one plane, as far as 

 the immediate region of the incisors, where it bends downwards for a short distance along the 

 incisors. Immediately anterior to the molars it contracts and continues very narrow. The 

 external boundary of the palate here is formed by a ridge, which diverges from its fellow opposite 

 the incisive foramen, and passes over on to the side of the snout along the maxillary suture. 

 The incisive foramina are quite large, though narrow, situated one-half in the maxillary, and 

 half in the intermaxillary. Their anterior extremity is midway between the molars and the 

 anterior base of the incisors, the entire length about equal to the three last molars. The width 

 of palate on either side of the foramina is scarcely greater than that of one of the incisive 

 foramina themselves. On the posterior edge of the palate there is an angular notch at either 

 side, which extends nearly to the second molar. 



The inner lines of the molar alveoli are perfectly straight, but convergent, so that they would 

 intersect a little in advance of the incisors. The external lines are convex. The total length 

 of the four upper molars is only from one-eighth to one-ninth of the entire length of the skull. 

 The molars themselves, four on each side, above and below, are prismatic and rootless. They 

 are inserted in a convergent manner, so that the axes of the first and last would intersect at 

 from one-fourth to one-half an inch from their crowns. The crowns of the molars are trans 

 versely elliptical, diminishing from first to last, which is subcircular or trigonal. In the old 

 and worn tooth, there is a simple compressed cylinder of enamel, enclosing dentine, the centre 

 of which is hardest. There is also a slight groove on the outer edge of the crowns of both upper 

 and lower. In the unworn full grown tooth, there is a distinct notch in the middle of the 

 exterior edge of both upper and lower molars, caused by the indentation of the enamel. This 

 indentation is but slight, and becomes ground away, until we have, in old specimens, the 

 perfectly simple external cylinder of enamel, as in some edentates. On the anterior molars of 

 both jaws, however, the indentation is on the inner edge. 



The first, or deciduous premolar, remains till very late in life, and is distinctly rooted, its 

 successor, however, being rootless like the rest. It is considerably larger than its successor, 

 and has a rather close resemblance to the first molar of the genus Mus, in the three transverse 

 tuberculated ridges. 



The incisors are very small and delicate. The upper incisors have their root just under the 

 anterior extremity of the ridge separating the anterior face of the zygomatic process of the 

 maxillary from the superior, and their course is easily traceable by the bulging out of the sides 

 of the muzzle. The anterior face is deeply grooved, its outer half, at the same time, being 

 rabbeted out, so that the groove is visible with equal distinctness from in front and on the sides. 

 The corners, too, of the tooth are rounder. The lower incisors are rounded, but not grooved. 



The skeleton of Dipodomys ordii has 7 cervical, 12 dorsal, 9 lumbar, 4 sacral, and 28 caudal 

 vertebra) =. 60. The second, third, and fourth cervical are firmly anchylosed together, with r- 

 joint upper spinous process. The hind legs are disproportionately elongated ; the metatarsa 



