MURID^— 8IGMODGNTBS— NEOTOMA. 



11 



apprcsscd, and thus, as one double enamel fold, they penetrate the tooth. 

 The apex of the antero-exterior Ioo|) in each upper tooth abuts against the 

 apex of the single interior loop; the apex of the other exterior loop abuts 

 against the surrounding wall of enamel of tiic opposite side; and thus the 

 crown is divided into three distinct dentine islands. But, in other cases, these 

 reentrant promontories of enamel do not reach either the internal loop or the 

 inner wall; then we have a straggling djntine area on the face of the tooth 

 partially cut up into three. The faces of tlie crowns difl'er in sculpture to a 

 considerable degree with age, as in other genera ; but, owing to the more pris- 

 matic form of the teeth, this variability is not so great as in Ilesperomys 

 or Mus, and the pattern above described will be found essentially preserved 

 in most cases, except in the extremes of youth and old age. In very worn 

 leeth the reentrant folds are all ground out, leaving a single irregular dentine 

 area surrounded by a. crenulate enamel wall, as in Sigmodon, &c. ; at one 

 period, there are enamel islands left in this area. The back molars wear down 

 from the condition above described to a simple figure-of-8, or hour-glass 

 shape. The under molars differ from the upper in being more complicated, 

 but also vary among themselves to a greater degree still. In the upper, 

 moreover, the lateral protuberances are essentially alternate (there being two 

 outside and only one inside) ; while in the lower, there being the same num- 

 ber of indentations on both sides, these are opposite. The front an.l middle 

 lower molars are extremely similar to each other, both having two reentrant 

 loops and three rounded saliencies (all mutually opposite), so that their crowns 

 make a figure-of-8, with an extra loop. The chief difference between them 

 is that the anterior lobe of the front one is enlarged a little, and shows a 

 shallow indentation. The apices of the lateral reentrant loops abut each 

 against its fellow, by which means the crown is divided into three dentine 

 spaces, one behind the other ; often, however, the ends of the loops do not 

 touch, so that passage-way is left from one dentine space into the others. 

 The hinder tooth may be best described as a two-thirds of either of the 

 others : i. «., it is a figurc-of-8 without the extra loop above mentioned ; it has 

 one reentrant fold of enamel on each side, opposite the middle. These folds 

 may meet or not in the center of the tooth ; and, in very old teeth, the grind- 

 ing-away of the folds leaves simply a somewhat irregular subcircular crown. 



One specimen shows an instructive condition of this last under molar. 

 The exterior reentrant loop is becoming detached from the side-wall of enamel, 



