Family CASTOROIDID^. 



Genus CASTOROIDES Foster. 



Caitoroidet Fohtri), H«ooud Kop. Oeol. of Ohio, 1838, 81.— Wyman, Boston Jonrn. Nat. Hist., t, 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 fur 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 fronted. The niirrowes*; 

 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 bullse 

 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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