48 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
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 Arvicoline, where the short solid skull and 
massive jaws and deep muscular impressions and prominent bony points 
dappui, 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 NMeotoma or Sigmodon or Mus are 
hardly more inferior in power to the ever-growing heavily-mailed grinders of 
Arvicoliné 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 Vespertmus 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, 
between 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. 
