HAPLODONTIDA—TEETH OF HAPLODON RUFUS. 563 
the last to the next to the first in the upper jaw, from last to first in the lower. 
In the upper jaw, the crenate border of the teeth is interior, the straight 
spurred border exterior; in the lower jaw the reverse. All the molars are 
rootless and prismatic, as in Castor, but not in other Sciuromorphs. The 
dental formula is as usual in Sciuride ([=3), but not asin Castor. The pattern 
of the molar crowns is simple, not complicated, as in (all?) other Sciuromorphs. 
In the upper jaw, the anterior premolar is very small, and otherwise different 
from the rest of the teeth; the other premolar resembles the true molars. In 
the lower jaw, all the teeth are similar to each other. 
The anterior upper premolar is a small simple cylinder, lying very 
obliquely against the antero-interior corner of the succeeding tooth; an oblique 
bevelling of its face remedies the obliquity of the shaft, causing the plane of 
the crown to coincide nearly with that of the other teeth. The other pre- 
molar and the three molars proper may be most conveniently described 
together, afterward noting a slight peculiarity of the former. These teeth 
are set with strong obliquity outward; they regularly decrease .in size 
from before backward; the shape of the crowns is substantially the same, 
and presents a pattern probably unique. The horizontal section of each 
tooth gives a half-elliptical or semicircular figure, with a prominent spur pro- 
jJecting from the straight side. The spur is exterior, the convexity of the 
half-ellipse interior; so that the molar series, taken together, presents a 
crenate inner margin, and a straight outer margin with four equidistant 
projections. The hindmost tooth is semicircular; the increasing width of 
preceding teeth changes this into the semi-elliptical shape, the anterior tooth- 
being further modified by a slight emargination where the small anterior pre- 
molar abuts against it, and further by a slight concavity of the straight outer 
border on each side of the spur. In some specimens, the regularity of the 
semicircular or elliptical curves is interfered with; and the back premolar 
may show, in addition to the emarginations just noted, an emargination of the 
antero-exterior corner. 
The lower series of molars substantially repeats the figures of the upper 
in the reverse direction, the spurs and straight edges being interior, the con- 
vexities exterior. For the rest, the four teeth (1 Pm., 3 M.) differ less in 
size among themselves than those of the upper series do; the spurs are much 
less prominent, and the sides of the teeth from which they spring are not so 
straight; the regularity of the convexity of each tooth suffers from an emar- 
