394 Hall and Wyman on 
0.6 inch — unlike the Castors, Ondatras, and Capybaras, it is 
destitute of an emargination on its upper border. The occi- 
pital condyles are semi-terminal, forming a gynglimoid articula- 
tion with the atlas, which allows a free vertical motion on the 
vertebral column, but precludes the possibility of any buta 
very slight one in a lateral direction. The basilar portion of 
the occiput has a slight ridge on the median line, and at its 
union with the sphenoid bone there exist two conical projec- 
tions, united by a transverse ridge, and are codssified with the 
inner pterygoid processes, thus forming a part of one of the 
entrances to the posterior nares. ; 
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 pro 
cess having a concavity forwards, which enters into the forma- 
tion of the posterior limit of the pterygoid fossa. The 
external auditory meatus, like that of the Castors, consists 
of a long tubular process, about an inch in length, and ex- 
tending upwards and forwards in a curved direction; the 
external orifice of which scarcely exceeds that of the common 
aver. 
In the development and conformation of the pterygoid ps 
cesses, the Castoroides differs from all the existing Rodentia. 
Both processes articulate with the tympanic bone, but the de- 
velopment 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 corresponding process of the opposite side. In 
consequence of this, the entrance to the posterior nares, or e 
meso-pterygoid fossa, is completely obstructed in its mid : 
portion, and instead of one large quadrangular orifice, as 12 
other Rodentia, we have two distinct orifices ; one of the 2x 
superior, of a pyriform shape, the circumference of which 15 
ormed in part by the posterior extremities of the pterys°” 
processes, and in part by the anterior or basilar portion of the 
