50 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
the tooth, surrounded by a zigzag enamel-wall, which sends into the dentine 
space, from each side, two indentations ; these indentations on the outer side 
being much deeper than those from the inner side, and semicircular in out- 
line, with convexity forward; these insulations being loops of enamel, 7 e., 
consisting of the enamel-sheet folded against itself; at the point of the begin- 
ning to fold, there is, of course, an external nick or reéntrance, and so there 
are two of these on each side of the tooth, the inner being the more open. 
In the next stage, a little further abrasion grinds out the continuity of these 
inlying enamel-folds with the general enamel-envelope, because the folds are 
not so deep down in the substance of the tooth at its edges as they are in the 
interior; and then we have the condition of crescentic islands of enamel lying 
in the general dentine area that is surrounded by the general indented enamel- 
wall. This occurs at full maturity. The final condition of senile decline is 
still something different; for, lastly, these enamel islands are entirely rubbed 
out, and the face of the tooth is one continuous area of dentine, a little exca- 
vated or sunken below the level of the continuous exterior sheet of enamel 
that irregularly surrounds it. 
These special details, though readily observed, are difficult to describe 
clearly, and the description must be followed with specimens in hand. 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 net-work of crust (enamel) ; 
other slices will make a continuous hole through the crust (enamel), exposing 
a continuous area (dentine) bounded by an irregular wall of crust; and 
So on. 
All this is very different from the straight upright bundles of prisms that 
compose the teeth of Arvicola; after the bumpy tops of which are once filed 
down smooth, further abrasion, continued never so long, does not essentially 
modify the pattern of the crowns. 
HESPEROMYS (VESPERIMUS) LEUCOPUS. 
White-footed or Deer Mouse. 
American Field Mouse, or Rat, PENNANT, Synopsis, 1771, No. 303; Hist. Quad. 1781, No. 302; Aret. Zool. i, 
1784, 131. 
American Wandering Mouse, Barton, Med. & Surg. Journ. Phila. i, 1805, 31 (notices a great migration 
by Lake Erie). 
