MAMMALS OF MINNESOTA. 177 



The lower jaw is elongated, with low coronoid and broad but 

 not hamular angle. The skull is twice as long as wide. The 

 length of the lower jaw is about three times its hight. 



The scapula differs from that bone in Arvicola by being less 

 slender and having a short acromium and broad metacromium. 

 The deltoid ridge of the humerus is fairly developed. The 

 sternum consists of six sternebrae, the manubrium being very 

 broad anteriorly. The fibula is united with the tibia, as in 

 Arvicola, but is less slender, and the limb is proportionally 

 longer. 



We have purposely omitted the dentition from the above 

 account, preferring to quote Coues' statements as the most sat- 

 isfactory general account at hand: 



''The molar series is both short and narrow, between one- 

 sixth and one-seventh the length of the skull. . . . The molars 

 rapidly decrease in size from before backward, particularly in 

 the upper jaw, where the last one is subcircular, and not more 

 than half as large as the middle one, which itself is less than 

 the front one. . . . The molars of the upper jaw'have three 

 roots apiece, two external and one internal; those of the under 

 jaw have but two, placed one after the other on the median line. 

 . . . The unworn molars of Hesperomys show a double 

 lengthwise series of conical tubercles connected by lower cross- 

 wise ridges, and the whole face of the tooth is encased in a 

 sheet of enamel continuous with that of the sides of the tooth. 

 . . . The tubercles are not exactly opposite each other in 

 crosswise pairs, but are half -alternating. Down between the 

 bases of these conical eminences are seen furrows, the more 

 readily noticeable because generally blackened, apparently by 

 the sticking of foreign matter in them. They represent the 

 deep, close-curved plications of enamel that penetrate the tooth 

 from either side, the ends of the loops nearly or quite meet- 

 ing in the substance of the tooth. ... It will be seen that, 

 after abrasion has commenced, the molar crowns will present a 

 different pattern with each stage of the process. . . . The 

 student may imagine the top of a pigeon pie, full of humps and 

 hollows, gradually razeed down by a succession of thin parallel 

 horizontal slices. Let the crust be the enamel, and the substance 

 of the pie the dentine ; the first slice will shave off the tops of 

 one or more humps, exposing the interior (dentine) in isolated 

 places, these islands lying in a network of crust (enamel)." 



The habits seem to be as uniform as the structure, and our 

 species may furnish an idea of the group. 



