Famy CASTOROIDID A. 
GENUS CASTOROIDES Foster. 
Castoroides FostER, Second Rey. Geol. of Ohio, 1838, 81.—WyMan, Boston Journ. Nat. Hist., v, 1846, 401. 
The skull, in general outline, considerably resembles that of Castor, 
‘but the cranial portion is relatively very much smaller and more flattened, 
and the facial portion much longer than in that genus. The zygomatic 
processes arise at a much higher point, and the zygomatic arch is much 
less curved downward. The malar bone is narrower, relatively far less 
massive, and is less prolonged anteriorly, not reaching the front wall of the 
orbit. In Castor, on the contrary, it reaches the small lachrymal, by which only 
it is separated from the anteorbital process of the frontal. The narrowest, 
portion of the skull is behind the middle instead of anterior to it, as in Castor. 
The basilar cavity seen in Castor is entirely wanting, and the auditory bulls 
are smaller. ‘The tympanic portions of the temporal bones present very 
nearly the same conformation as in the Capybaras; at the inner extremity, 
however, there exists a broad plate or process having a concavity forward, 
which enters into the formation of the posterior limit of the pterygoid fossa. 
In the development and conformation of the pterygoid processes, the 
Castoroides differs from all existing Rodentia. Both processes articulate with 
the tympanic bone, but the development of the external plate is by far 
the greatest; the internal, however, has the remarkable peculiarity of being 
curved inwards towards the median line, so that the most prominent part 
of its convex surface is brought in contact with that of the correspond- 
ing process of the opposite side. In consequence of this, the entrance to 
the posterior nares, or the meso-pterygoid fossa, is completely obstructed in 
its middle portion, and instead of one large quadrangular orifice, as in other 
Rodentia, we have two distinct orifices; one of these, superior, of a pyriform 
shape, the circumference of which is formed in part by the posterior extrem-: 
ities of the pterygoid processes, and in part by the anterior basilar portion of 
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