372 The Extinct Rodentia of North America. [Apri 
molar teeth of Castoroides are strictly comparable with those of 
the Chinchillide, and with the posterior portion of the last molar 
of Hydrochzrus, and thereby differ not only radically from that 
seen in Castor, but from that of all other rodents. 
“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 de 
scending ramus of the lower jaw, the latter a character shared 
also by Fiber. The differences consist in the remarkable struc 
ture of the pterygoid processes, the double orifices 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 one hand to the Chinchillas, from 
which it differs mainly through those points in which it resem- 
bles Castor. In view of tĦese wide differences from its nearest | 
well-known allies, it seems to constitute the type of a distinct and | 
hitherto unrecognized family. To the same group are, howev® 
probably referable the genus Amblyrhiza, described by Professor 
Cope, from the bone caverns of Anguilla island, West Indies. 
These forms are thus far known only from the detached teeth and | 
fragments of the limbs. The molars as described and figured by 
Professor Cope, greatly resemble those of Castoroides, having ? 
act the same structure, differing mainly in being 50% 
smaller, and in possessing a greater number of lamin@. ( 
are also other differences, see below under Amblyrhiza.) | 
“ The Castoroides ohioensis was of about the size of a eee 
r 
í 
ing in size the Capybara, the largest of existing ro mee 
of a skull has a length of over twelve inches. The spea b 
known only from a few cranial and dental remains, it 15 " pabis. 
ble to say much respecting its general form or probable reet 
It may have been aquatic like the beaver; but of he 
no evidence. The form of the occipital condyles and the vobably 
for the attachment of the cranial muscles show that it PE 
differed greatly in habits from the beaver.” we egth 
The sculpture of the incisor teeth of this species * = 
and distinguishes it readily from all other Rodentia. 
SCIURIDÆ. 
i t Teol 
Squirrels were probably abundant during the — a 
