Castoroides Ohioensis. 397 
On the inner face of the bone, the fossa, serving for the inser- 
tion of the inner pterygoid muscle, occupies the whole of the 
angle of the jaw, the surface of which is much increased by the 
development, on its edge, of the thin plate of bone which ex- 
ists in the Castors, but is very slight. A well-marked fossa is 
also noticeable at the base of the coronoid process, and a well- 
defined oval impression 2 inches long, and 1 inch broad, situ- 
ated just below the molares, indicates the existence of a pow- 
erful mylo-hyoid muscle. The muscular depression at the 
symphysis indicates a corresponding power in the digastricus 
and genio-hyoideus. 
From the above descriptions it will be seen that in its oste- 
ology the Castoroides has greater analogies to the Castors than 
to any other genus of Rodents, but differs materially from 
it, however, in the forms of those parts which serve as origins 
to the muscles of mastication. It now remains to examine 
the structure of the teeth, and to institute comparisons between 
them and those of the allied genera. 
The incisors have been already described by Mr. Foster, in 
his notice of the lower jaw ; they have a triangular form, one 
of the faces presenting forwards, and one of the angles back- 
wards. The enamel on the two lateral or posterior faces is 
smooth and thin, while that on the anterior is much thicker, 
and deeply grooved or fluted, the grooves corresponding with 
Others less distinctly marked on the surface of the dentine. 
The alveolus lodging the incisor of the lower jaw extends as 
far back as the angle, and the whole tooth has a length of be- 
tween 10 and 11 inches. The superior incisors have curves 
lesser radii, and are much shorter, but are similarly grooved 
on the anterior face, and are exposed for the distance of about 
three inches. In none of the existing genera are the grooves 
the enamel so distinctly marked. : 
. The molares form a continuous grinding surface in both 
Jaws, that of the upper slightly convex, 2.5 inches in length, 
tof the lower concave having a length of 2; inches, and 
