50 MONOGRAPHS OP NORTH AMERICAN RODENTlA. 



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, i. 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 reentrance, 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 Ihese 

 inlying enamel-folds with the general enamel-envelope, because the folds are 

 nol so deep down in the substance of the tooth at its edges as they arc 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 oil' 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 ; alter the bumpy tops of which are once filed 

 down smooth, furl her abrasion, continued never so long, does not essentially 

 modify the pattern of the crowns. 



HESPEROMWS (VESPERIMUS) LEUCOPUS. 



White-footed or Deer Mouse. 



American Field Mouse, or /.'«/, Pennant, Synopsis, 1771, No. :so:S; Hist. Quad. 1781, No. 302; Arct. Zool. i, 



1784, 131. 

 American Wandering Mouse, Bakton, Med. & Surg. Journ. Pbila. i, 180"), :!1 (notices a great migration 



by Lake Brie). 



