MURIDA—SIGMODGNTES—NEOTOMA. nt 
appressed, and thus, as one double enamel fold, they penetrate the tooth. 
The apex of the antero-exterior loop 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 the opposite side; and thus the 
crown is divided into three distinct dentine islands. But, in other cases, these 
recntrant promontories of enamel do not reach either the internal loop or the 
inner wall; then we have a straggling dentine area on the face of the tooth 
partially cut up into three. The faces of the crowns differ 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 Hesperomys 
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 
teeth the reéntrant folds are all ground out, leaving a single irregular dentine 
area surrounded by a crenulate enamel wall, as in Sigmodon, &e.; at ove 
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 and middle 
lower molars are extremely similar to each other, both having two reéntrant 
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 reéutrant 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: 7. ¢., it is a figure-of-8 without the extra loop above mentioned; it has 
one reéntrant 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 reéntrant loop is becoming detached from the side-wall of enamel, 
