400 Hall and Wyman on 
oped in this animal, than in the members of any of the allied 
genera of Rodents: they are indices of the great force with 
which their powerful incisors may be used. 
The well-marked depressions which indicate the insertions 
of the mylo-hyoid, digastric and genio-hyoid muscles are also 
interesting. The functions of these muscles are twofold ; first, 
to elevate or bring forwards the os hyoides, as in the act of deglu- 
tition, the lower jaw being a fixed body ; this, however, requiring 
but a very moderate amount of muscular force: second, to de- 
press the lower jaw, which they can do only when the os hyoides 
is rendered immovable by the action of the sterno-hyoid and 
sterno-thyroid muscles. It is with reference to this last func- 
tion, the depression of the jaw, that the muscles in question 
are so remarkably developed, and thus supply a powerful an- 
tagonistic force to that which moves the jaw in the opposite 
direction. This force would frequently be brought into play 
in disengaging the teeth, when firmly imbedded, as must 
sometimes happen, in the woody substances which they were 
ghawing or cutting. 
The great length of the portion of the incisor teeth im- 
bedded in the alveoli, is scarcely less remarkable than the 
other peculiarities of this skull. The final cause of the great 
length and the curved form of the incisive teeth of the Cas- 
toroides as well as of the Rodents, in general, would seem to 
be twofold ; first, to increase the surface of the attachment of 
the tooth, and thus afford more points of resistance to the 
pressure applied to its free extremity during the ordinary Use; 
secondly, the curved form serving to transmit that pressure to 
the convex surface, instead of the base of the tooth; this 
last being always in a growing condition, is from necessity 
pulpy as well as highly vascular, and, therefore, incapable of 
sustaining any great degree of pressure. 
Though a matter of great interest, it is hardly possibly to 
form a very correct estimate of the size of the Castoroides, 
almost the only data offered to us being those given by the 
cranium. The length of the skull of this animal is 10.5 
