Castoroides Ohioensis. 399 
On reviewing the description of this cranium we find that 
it presents analogies to the genera Castor, Fiber and Hy- 
drocherus. Osteologically considered, the cranium bears a 
stronger resemblance in its shape to that of the Castors, than 
to either of the other genera; but in its dentition the type is 
wholly different, as is also the conformation of the pterygoid 
processes and fossx. 
Compared with the Castors, the relative capacity of the 
cranium is much smaller, and the occiput more depressed ; 
the occipital condyles admit of a free and extensive motion 
vertically, but only a very limited one in a lateral direction ; 
the foramen magnum has a depressed oval form, and is des- 
titute of an emargination on its upper edge. It differs from 
all other Rodents in the size and conformation of the ptery- 
goid processes and fosse ; especially in the incurvation of the 
internal processes and the consequent subdivision of the en- 
trance to the posterior nares. It differs entirely from the 
Castors in the compound nature of the molar teeth, and in 
the flutings of the incisors ; in the diminutive size of the inci- 
sive foramina, and in the conformation of the lower jaw with 
reference to the insertion of the muscles of mastication. 
In the Hydrocherus the principal analogies are found in 
the Compound nature of the molar teeth, from which those of 
the Castoroides, however, are readily distinguished by the 
Posterior molars of the former having an increased number of 
hamine ; by the complication of the anterior molars in conse- 
quence of the involution of the enamel on the inside of the 
teeth"of the upper, and the outside of those of the lower jaw, 
and by the serrated edges caused by the existence of a small 
quantity only of crusta petrosa between the la is * 
In the Fibers the pterygoid fosse are largely developed, à 
the entrance to the posterior nares has the same conformation 
as in other Rodents. Sy $ 
. All the fossæ and processes which serve as origins or inser- 
S to the muscles, (and consequently the muscles — 
selves.) of the lower jaw, are much more remarkably devel- 
