48 MONOGRAPHS OF NORTH AMERICAN RODBNTIA. 



so Far back, terminating nearly or exactly opposite the last molars (see Ory- 

 zomys). The lower jaw is straighter on the whole: than in many Murines, 

 from the great backward set of its condylar ramus; but in spite of this 

 obliquity, the coronoid is so short (a mere little sharp point of bone) that it 

 does not attain the level of the condyle. The outside of the ramus is strongly 

 ridged by the passage of the incisor-root. The descending process of the 

 lower jaw is rather slight, subtriangular, with a sharp termination, lies below 

 the level of the molars, and has its under edge inflected. 



Before noticing the dentition, we may pause to observe what indication 

 of the habits and food of the animal the bare skull affords us, without refer- 

 ence to the teeth. The comparative weakness of the whole masseteric 

 arrangement is evident. It is a wide remove from the climax of rodent 

 masticatory apparatus seen in the Arvicolince, where the short solid skull and 

 massive jaws and deep muscular impressions and prominent bony points 

 d'appui, are all so conspicuous. There is even less of this sort of thing than 

 is seen in Mus or Neotoma or Sigmodon. The thinness and smoothness of 

 the skull, and the comparatively slight bony points it develops, prepare us 

 for the very modest dental armature that we find, and clearly indicates a diet 

 of much softer substances. The teeth of Neotoma or Sigmodon or Mus are 

 hardly more inferior in power to the ever-growing heavily-mailed grinders of 

 Arvicolince than are the teeth of Hesperomys to those of the genera just 

 named. Among American forms, no one except Ochetodon has such small and 

 weak molars as Vesperimus shows. 



The incisors offer nothing specially noteworthy. The upper are short, 

 much curved, narrow across, a good deal deeper than broad, smooth in front, 

 and shortly obliquely beveled behind ; the under are much longer and slen- 

 derer, and with longer beveling. The molar series is both short and narrow, 

 lict ween one-sixth and one-seventh the length of the skull, and thus hardly 

 longer than the distance between them, or the width of the palate. 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 ; in the lower jaw, 



the same progressive diminution occurs, but the difference in size is not quite 

 so evident. The molars of the upper jaw have three roots apiece, two exter- 

 nal and one internal ; those of the under jaw have but two, placed one after 

 the other on the median line. 



