230 MONOGRAPHS OF NOKTII AMERICAN RODBNTIA. 



t he incisor-root up the ramus results in, and is betrayed by, a prominent super- 

 ficial ridge or thickening thai runs from the inside of the hack of the condylar 

 ramus downward, forward, and a little inward, to connect with the posterior 

 extremity of the alveolar ridge. In Myodes, Cuniculus, and Synaptomys, this 

 ridge is wanting; the whole condylar ramus being thin and flat, its inner iace 

 nearly plane, and separated from the posterior end of the alveolus by a strong 

 sulcus. Moreover, the passage of the incisor-root so closely along the under 

 holder of the body of the jaw confers upon the latter a massiveness not seen 

 in other genera. The protruding ends of the incisors offer nothing peculiar. 



There is not much else to note in the lower jaw. The apex of the coro- 

 noid reaches nearly or quite to the level of the condyle, as in Arvicola gener- 

 ally. The descending hamular process, as in Myodes, is large, strong, curves 

 strongly outward, and has a much inflected lower border, so that its inferior 

 aspect is a flat surface instead of a border ; and the notch between it and the 

 back of the condylar ramus is wide and deep. The muscular impressions 

 upon the side of the jaw are deep and strong, bounded by prominent sharp 

 ridges. 



As is well known, the molar teeth of Myodes (as restricted to exclude 

 Cuniculus) are both individually and collectively distinguishable on sight from 

 those of other Arvicolinee; for, although constructed essentially upon the same 

 plan of aggregated rootless prisms, their details are quite different. In ordinary 

 Arvicoltnce, the molars as a series are sharply serrate, both internally and exter- 

 nally, by reason of the acute salient and reentrant angles they present from first 

 to last, both above and below. Now in Myodes, of the upper molars the outer 

 saliencies are similarly sharp, but the outer reentrances are so deep that they 

 reach almost across the teeth to the inner side; while the inner saliencies are 

 so obtuse and the inner reentrances so shallow that the inner border of the 

 series as a whole is crenate instead of serrate; and in Myodes, again, of the 

 under molars, the inner saliencies are sharp and the inner reentrances deep, 

 while the; outer saliencies are so obtuse and the outer reentrances so shallow 

 that the outer border of the series as a whole is crenate instead of serrate. 

 Thus, in Myodes, one border of both upper and under molar series is crenate, 

 the other serrate, the crenation being external in the under molars, and 

 internal in the upper molars. Now this molar pattern is repeated in Synap- 

 tomys with such fidelity that we can indicate no difference of generic import; 

 and thus the great difference between the molars of Synaptomys, and of 

 Arvicola with its subdivisions, as well as of Evo'omys, becomes evident. 



