CASTOEOIDID.E— CASTOROIDES— C. OHIOENSIS. 421 



of the skull also somewhat resembles that of Lagostomus, but differs in 

 important details. Castoroides, for instance, lacks the large antorbital vacuity 

 seen in Lagostomus and its allies; this portion of the skull more resembling 

 that of Castor. The lower jaw also much more resembles that of Castor, 

 b'ut differs from that of the latter in having the coronoid process much less 

 strongly developed; in the much greater elongation of the condylar process; 

 in the great depth of the fossa on tbe outer surface below the notch formed 

 by the coronoid and condylar processes (which is shallow in Castor); in the 

 lateral flattening of the condyles; in the bending inward of the angular 

 process and its much greater development, as well as in the much greater 

 depth of the fossa on the inner border of the lower surface of the angle. The 

 inward curvature of the angular process is also an exceptional feature among 

 Rodents. Castoroides further differs from Castor in the form of the occipital 

 condyles, which are more terminal and less oblique than in Castor, and, while 

 permitting a great range of vertical motion of the head, allow only a slight 

 lateral motion. 



From the foregoing remarks, it will be seen that Castoroides presents a 

 singular combination of characters, allying it, on the one hand, to the Beaver, 

 and, on the other, to the Chinchillas and Viscachas, and also to the Muskrat, 

 but which, at the same time, separate it widely from either group. In size, 

 Castoroides exceeded any living Rodents, and is itself exceeded in this order 

 by only a single extinct form of Hydrochcerus, described by Dr. Lund from 

 the bone-caverns of Brazil. The resemblance of Castoroides to Castor is 

 mainly in the general outline of the skull, in its having an imperforate ante- 

 orbital wall, and in its presenting a similar curvature of the descending ramus 

 of the lower jaw, the latter a character shared also by Fiber. The differences 

 consist in the remarkable structure of the pterygoid processes, the double 

 orifice of the posterior nares being entirely exceptional ; in the flattened and 

 relatively small cranium ; and in the compound nature of the molar teeth. 

 These differences ally it, on the other hand, to the Chinchillas, from which it 

 differs mainly through those points in which it resembles Castor. In view of 

 these wide differences from its nearest well-known allies, it seems to consti- 

 tute the type of a distinct and hitherlo unrecognized family. To the same 

 group are, however, probably referable the genera Ambhjrhiza and Loxomy- 

 lus, described by Professor Cope,* from the bone-caverns of Anguilla Island, 



* Proc. Acad. Nat. Sci. Phila . 1868, 313 ; Pioc. Amer. Phil. Soc, vol. xi (1869-70), pp. ISO, 608, 

 pis. iv, v. 



