10 



MOXOGBAPnS OF NORTQ AMERICAN KODEN'TIA. 



or space between tliese and the penultimate pair. The incisive Ibramiiia are 

 of nearly usual size and sliape; liicy do not quite rcacii to the molars. The 

 under jaw is noticeable for the great size of the coronoid process, whicli over- 

 tops the condyle. The descending process is large, subquadrate, and flaltish, 

 with (lie under edge thickened and curled inward. 



AVith a general resemblance to that of Sigmodon, the molar dt^ntition 

 of Ncotoma exhibits a meutionable tendency to recede from the ordinary sig- 

 modont style, and approach the arvicoline, in the somewhat prismatic nature 

 of the extra-alveolar j)art of the teeth. The teeth, however, are firmly rooted, 

 and the arvicoline bent is at\er all little more than a superficial resemblance. 

 The upper teeth are 3-rooted, as usual in the tribe, ciich with two exterior 

 and one interior fang; but the anterior two of these arc ollen 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 exi.st after the teeth are 

 extruded from the gums; and, at any rate, this ftict indicates a tooth that 

 grows much more ra))idly than in Mus, Ilesperoinys, or Oc/ielodon. It is much 

 tiic same with Sigmodon as witii N^eotoma. The teeth, as in Ilesperomys, &c., 

 decrease in size from first to last, in both jaws, though in this case there is 

 less diirerencc; 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 llie teeth as follows: All the 

 upper ones are trilobate externally, bilobate infernally ; that is to say, there are, 

 upon the outer side, two deep, opdn Indentations, where the eiuimcl-sheet loops 

 into the tooth, and, consequently, three rounded saliencies or h)bes, 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 autero-interior lobe is slightly indented, making 

 three lobes in all, as on the outside. The interior reentrant loops of enamel arc 

 wide open and sliallow, not reucliiug half-way across liie face of the tooth; 

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

 iiCHLss Ihe tooth. At tiic outset, these exterior loops are wide open, like the 

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



