} 
CASTOROIDIDH—CASTOROIDES—C. OHIOENSIS. 421 
of the skuil 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, 
but differs from that of the latter in having the coronvid process much less 
strongly developed; in the much greater elongation of the condylar process ; 
in the great depth of the fossa on the 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 weil as in the much greater 
depth of the fossa on the inner border of the lower surlace 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 Hydrocherus, described by Dr. Lund from 
the bone-caverns of Brazil. The resemblance of Custoroides 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 hitherto unrecognized family. To the same 
group are, however, probably referable the genera Amblyrhiza and Loxomy- 
lus, described by Professor Cope,* from the bone-caverns of Anguilla Island, 
*Proc. Acad. Nat. Sci. Phila., 1868, 313; Proc. Amer. Phil. Soc., vol. xi (1869-70), pp. 153, 608, 
pls. iv, v. 
