HAPL0D0NTIDJ3— TEETI3 OF H.\PLODON RUFTJS. 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 Sciutidcn (f^), but not as in 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- 

 jecting 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 cmargination 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- 



