THE BEAVEK. 
155 
behind forwards and vice versa. They have no true 
roots, but are of equal thickness throughout, and are 
implanted within the jaw in sacs or capsules, which 
reproduce them from the base as fast as they are worn 
down at the extremity. So strong a tendency have 
they to increase by this process, that whenever one of 
the incisors of either jaw has been accidentally injured 
or destroyed, the opposite tooth, meeting with no resist- 
ance from its antagonist, is propelled forwards by a 
continual enlargement from the base, to such an extent 
as to become at length perfectly monstrous. This mode 
of growth is common to the whole Order; and the 
number of the incisor teeth is also the same in all the 
groups that compose it, with the exception of the Family 
of which the Hare forms the type. 
The entire absence of canine teeth, leaving a vacant 
space of some extent between the incisors and the 
molars, is another character which the Beavers have 
in common with all the Rodent animals ; but the struc- 
ture of their molar teeth differs from that of any other 
group. These latter organs furnish indeed the best 
characters that have yet been employed for the sepa- 
ration of the Rongeurs into distinct and natural genera. 
In the Beavers they are four on each side in either jaw, 
and their crowns present a flattened surface on which 
the lines of enamel are so disposed as to form three 
folds on the outer side and one on the inner in those 
of the upper jaw, while those of the lower offer an 
arrangement directly the reverse. They were formerly 
suspected by M. F. Cuvier, who has paid particular 
attention to the teeth of the mammiferous quadrupeds, 
to be destitute of proper roots, and to increase from 
their base in the same manner as the incisors ; but he 
has since candidly confessed the error into which he 
had been led by the inspection of a cranium in which 
