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- 

 toils the condyle. The descending process is large, subquadrate, and Saltish, 

 with the under edge thickened and curled inward. 



Willi a general resemblance to that of Sigmodon, the molar dentition 

 of Neotoma exhibits a mentionable tendency to recede from the ordinary sig- 

 modonl 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 resemblance. 

 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, seriatim. 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 Mils, 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 reentrant loops of enamel are 

 wide open and shallow, not reaching half-way across the lace of the tooth; 

 the exterior loops, on the contrary, are very deep, reaching nearly or quite 

 across the tooth. At the outset, these exterior loops are wide open, like the 

 interior ones; but they soon shut, the two folds of enamel being mutually 



