MURIDAE—SIGMODONTES—HESPEROMYS. 49 
In Hesperomys, as in Mus and Ochetodon, and not as in Sigmodon and 
Neotoma, the tubercles of the molar crowns are long persistent. A great 
majority of the specimens of Hesperomys in the collection before us present the 
tubercles intact, while it is rare to see skulls of Meotoma and Sigmodon in 
which the crowns are not already ground flat, so as to show the dentine area 
surrounded by the plicated enamel-sheet. This would seem to argue a much 
slower growth of the grinders. In the rapidly and continuously growing 
molars of Arvicola, the crowns are worn flat, and show their characteristic 
dentine triangles as soon as they fairly surmount the alveoli; here the oppo- 
site extreme is witnessed. The unworn molars of Hesperomys show a double 
lengthwise series of conical tubercles connected by lower crosswise ridges, 
and the whole face of the tooth is encased in a sheet of enamel continuous 
with that of the sides of the tooth. Although, as we have said, the main 
tubercles are biseriatim, yet the first pair of the front molar of either jaw may 
appear like one, from being so close together; this azygos anterior one being 
followed by two perfectly distinct pairs; the second tooth has only two pairs, 
but perfectly distinct ones ; on the small hinder tooth, the pairing of the tuber- 
cles is obscure. 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 rep- 
resent the deep close-curved plications of enamel that penetrate the tooth 
from either side ; the ends of the loops nearly or quite meeting in the substance 
of the tooth. 
If the foregoing account is perfectly intelligible, it will be seen that, after 
abrasion has commenced, the molar crowns will present a different pattern 
with each stage of the process. The main conical tubercles are first razeed, 
and then the connecting crests and little accessory tubercles follow by the 
same filing-down operation ; consequently, the pattern of the molar crowns 
must be used as a zoological character with great caution, if at all; the minor 
details are of no sort of consequence; and even in using this broad pattern it 
is necessary to compare age for age (or rather condition for condition, since 
different individuals get their teeth filed down with variable rapidity) in draw- 
ing up the characters of species or subgenera. At the last stage specified, 
namely, when main tubercles and connecting crests and accessory tubercles 
have disappeared, we see a single dentine area occupying the whole face of 
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