.Mil. 



CI . 



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230 



MONOGHAPnS OF NORTH AMERICAN RODBNTIA. 



the incisor-root up the ramus results in, and is hetrayed by, ii prominent super- 

 ficial ridge or thickening that runs from the inside of the back of the condylar 

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

 extremity of the alveolar ridge. In Myodes, Cuniculus, and Sj/naptomys, this 

 ridge is wanting; the whole condylar ramus being thin and Hat, its inner face 

 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 

 border of the body of the jaw confers upon the latter u massivencss 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- 

 noiil 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 Arvicolince; for, although constructed essentially uiwn the same 

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

 Arvicolina, 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 

 salicncies 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 taat the inner border of the 

 scries 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 tlie 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 Evolomys, becomes evident. 



