274 EXPLORATION OF THE CANONS OF THE COLORADO. 
the orbito-sphenoicl, lining the orbit behind, rises nearly to the top of the 
skull. 
The mandible remains for consideration. This is eminently character 
ized by its massiveness and the emphasis of its various ridges and angles. 
Nevertheless, the symphysis, though extensive, is incomplete. Instead of an 
edge below, the bone presents a broad, smooth, flattened area, bounded on 
the sides by a ridge indicating the limit of masseteric muscular attachment. 
The angle of the jaw -is strongly exflected in a peculiar way. An oblique 
plate (the "descending process" in many rodents) arises' from the inner side 
of the body of the bone, and curves strongly backward and outward, ending 
far exterior to the main part of the bone as a strong laminar process. Just 
inside of this, between it and the condyle, there is a strongly-marked, smooth, 
upright protuberance. This is where the root of the incisor pushes up from 
the inside. To the inner side of this knob, again, rises a third protuberance; 
it is the condyle, rather small, and of no noteworthy features. (It appears 
particularly small when compared with the glenoid cavity, which, as I should 
have remarked before, is of unusual width.) Thus the mandibles, viewed 
from behind, present the curious appearance of three prongs condyle, inci 
sor-knob, and exterior process. The appearance of trifurcation is best marked 
in Thomomys, where the tooth-knob is most prominent, and separated by 
deepest notches from the processes between which it stands. In addition to 
all these prominences, a slender, falcate, acute coronoid rises in front, and 
overtops the rest, being separated from the condylar ramus by a deep notch. 
There is a deep excavation between the thin laminar basis of the coronoid 
and the molar alveolus The foramen of the inferior maxillary nerve appears 
on the inner side of the root of the condylar ramus 
The dental formula has been already given. The molar dentition appears 
weak and slight in comparison with the enormous incisors. The under inci 
sors, as already said in effect, run the whole length of the jaw, and push up a 
knob of bone behind. They are of the ordinary scalpriform construction, 
quite flat-faced, with converging sides, and beveled to an edge behind. The 
superior incisors describe nearly a semicircle through the inter maxillaries, 
and far into the maxillaries, to below the root of the zygoma. They are 
