MAMMALIA, 197 
Gromys. (Rafinesque.) Sand-Rat. 
Geomys. ‘‘RaFINESQUE-SMaLTZz, Amer. Month. Mag, for 1817, p. 45,’ DEsmarEstT, Mamm., p. 314. 
Lesson. Man. de Mammal., p. 260. : 
Ascomys. LicHTENSTEIN ? Saccomys. F.CuviEr ? 
Pseudostoma. Say ? 
Prate xvut C, Fie. 1 to 6. 
CHARACTERS, 
0—0 
Dental formula, incisors, 3, canines =, 
grinders = = 20, 
Incisors strong ; linear and flattish anteriorly ; narrower posteriorly, and chamfered away 
evenly from their insertion into the sockets to their tips. The upper ones are generally 
marked with one or more grooves anteriorly; the lower ones have sometimes a faint groove 
on their exterior sides. The second and third pairs of grinders in each jaw are quite simple 
in their structure, each tooth consisting merely of a slightly curved cylinder of enamel, 
without roots, compressed from before backwards, with a longitudinal depression or shallow 
furrow on one side, which it renders more acute than the opposite one; the acute side of the 
grinders faces outwards in the upper jaw, and inwards in the lower one. The crowns of these 
teeth are flat, and have a transversely pear-shaped area, composed of soft bone, enclosed by 
a rim of enamel, but there are no transverse ridges, The posterior pair of grinders in each 
jaw are not so much compressed as those just described, but are nearly cylindrical, and have 
a roundish, slightly angular crown. The anterior pair, above and below, differ still more 
widely from the rest in being double, each of them being composed of two cylinders, 
shaped like the other teeth, and connected with each other by a narrow neck: the anterior 
cylinder is smaller than the other, and the long diameter of its crown is parallel to the 
axis of the jaw, and consequently is at right angles to the transverse pear-shaped crown of 
the posterior cylinder, and to the crowns of the teeth which succeed it, The upper grinders 
incline slightly backwards, the lower ones have a similar inclination forwards, and the grinding 
surfaces of both are very even. 
The lower jaw is particularly thick and strong, and its symphysis which slopes upwards 
nearly in the same direction with the incisors is about one-third of its whole length. The 
palate is very narrow, and in the scull exhibits a central longitudinal crest of bone, with a 
deep and partially covered furrow on each side of it for the passage of vessels. 
The head is large and depressed; the nose short. On examining the scull, the frontal 
and nasal bones are observed to be in the same plane, and the zygomatic arch is but a 
little depressed below the crown of the head. 
The nostrils are small round openings, facing downwards and somewhat laterally, separated 
