10 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
or space between these and the penultimate pair. The incisive foramina are 
of nearly usual size and shape; they do not quite reach to the molars. The 
under jaw is noticeable for the great size of the coronoid process, which over- 
tops the condyle. The descending process is large, subquadrate, and flattish, 
with the under edge thickened and curled inward. 
With a general resemblance to that of Sigmodon, the molar dentition 
of Neotoma exhibits a mentionable tendency to recede from the ordinary sig- 
modont style, and approach the arvicoline, in the somewhat prismatic nature 
of the extra-alveolar part of the teeth. The teeth, however, are firmly rooted, 
and the arvicoline bent is after all little more than a superficial resembiance. 
The upper teeth are 3-rooted, as usual in the tribe, each with two exterior 
and one interior fang; but the anterior two of these are often or usually 
(except in the first tooth) more or less fused together. The under ones have 
only two prongs, sertatim. The tuberculation of the molar crowns is an open 
question: we have never seen any teeth not worn flat, and cannot, therefore, 
speak of the character of the tubercles, if such exist after the teeth are 
extruded from the gums; and, at any rate, this fact indicates a tooth that 
grows much more rapidly than in Mus, Hesperomys, or Ochetodon. It is much 
the same with Sigmodon as with Neotoma. The teeth, as in Hesperomys, &c., 
decrease in size from first to last, in both jaws, though in this case there is 
less difference ; for the posterior upper one is at least two-thirds as large as 
the anterior one, and is but little less plicated. In the upper series, the 
decrease is regular from first to last; in the lower, the middle tooth is as 
large as the front one, but the back one suddenly diminishes in size nearly 
one-half. 
Average adult examples show a state of the teeth as follows: All the 
upper ones are trilobate externally, bilobate internally ; that is to say, there are, 
upon the outer side, two deep, open indentations, where the enamel-sheet loops 
into the tooth, and, consequently, three rounded saliencies or lobes, as just men- 
tioned; while on the inside there is one such indentation, or loop, opposite the 
middle of the tooth, producing two such rounded saliencies. On the front 
upper molar, however, the antero-interior lobe is slightly indented, making 
three lobes in all, as on the outside. The interior reéntrant loops of enamel are 
wide open and shallow, not reaching half-way across the face of the tooth ; 
the exterior loops, on the contrary, are very deep, reaching nearly or quite 
across the tooth. At the outsct, these exterior loops are wide open, like the 
interior ones; but they soon shut, the two folds of enamel being mutually 
