408 
U. S. P. R. R. EXP. AND SURVEYS-ZOOLOGY-GENERAL REPORT. 
outwards to an obtuse point. Tbe condyloid process is much compressed and quite deep, with 
upper and lower edges nearly parallel, and 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 tbe 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 o'f 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 
vertebrae = 60. The second, third, and fourth cervical are firmly anchylosed together, with a 
joint upper spinous process. The hind’ legs are disproportionately elongated ; the metatarsal 
