398 Hall and Wyman on 
elevated anteriorly so as to form an angle of nearly 45? with the 
body of the bone. The molars of both jaws diminish in size 
from before backwards, in which respect they differ from those 
of the Capybara. In the lower jaw the first molar has two deep 
grooves on the inner and one on the outer lateral surface ; 
the other three have a single groove on each side, so that the 
grinding surface of each tooth has something like an hour- 
glass contraction in the middle. In the upper jaw the reverse 
state of things exists, the last molar having the same peculiar- 
ities as the first in the lower. 
In their structure the molar teeth do not resemble those of 
the Castors, to which they have been compared. They are 
all compound, consisting, like those of the Capybara, among 
Rodents, and those of the Elephant among Pachyderms, of a 
series of laminze of dentine, invested with enamel, and united to- 
gether by means of an interposed ceementum or crusta petrosa. 
In the first molar of the upper and the last of the lower jaw, 
four such laminz exist, while in each of the others there are 
but three. The worn grinding surface presents a series of 
sections of these laminze, which are more or less contorted on 
the inner and outer border of the tooth, giving the appearance 
in some parts of the union of two adjoining laminse, but which 
does not actually take place in any instance. 
Thus we have teeth constructed. upon an entirely different 
plan from that of the castors, in which they are simple, the 
ridges on the grinding surfaces being formed. merely by invo- 
lutions of enamel, and not unlike that of the posterior molares 
of the Capybara, which consist of a series of laming, united by 
means of crusta petrosa. In the last-named animal, however, 
the number of laminz is thirteen, and the interspaces are 1M- 
perfectly filled with coementum, so that the edges of the teeth 
are more or less serrated ; but in the Castoroides the number 
of laminz does not exceed. four, and the crusta petrosa fills 
the whole of the interspaces. In the anterior teeth of the 
Capybara, there is an involution of the enamel at the edge 
which does not exist in the Castoroides; | 
