394 KANSAS UNIVERSITY SCIENCE BULLETIN. 
Castoroides, compared with Castor, is decidedly larger, a third 
deeper than wide, and occupies a more dorsal position pro- 
portionally, having a long, deep fossa extending cephalad and 
dorsad, which is wider and shallower anteriorly. In Cas- 
toroides the superior portion of the maxilla in the orbito- 
sphenoidal region is smoother, and does not have the excres- 
cences caused by the roots of the molars as in Castor. With 
the exception of a few of the foramina of Castoroides, all obtain 
a proportional enlargement over Castor. One very noticeable 
exception is the smal] size of the external auditory meatus. 
In this there is but a slight difference in the actual size be- 
tween the two, and is not nearly as large as that organ in 
Bizacacha. The tympanic bulle in Castoroides are very small, 
and but little inflated, while the basioccipital is compara- 
tively much broader and shorter. The double posterior nares, 
and the peculiarly constructed pterygoides, that have at- 
tracted the attention of all who have made a study of Cas- 
toroides, are beautifully shown in the Lenawee specimen be- 
longing to the Smithsonian Institute. (See plate 25, fig. A, 
at a-b.) The mastoid in Castoroides is an exceedingly stout 
and massive bone, compared with Castor; the outer edge of 
this bone and the sternomastoid are deeply scored and pitted 
for heavy muscular attachment. The glenoid fosse in Cas- 
toroides are shallow and broad, and would allow a more lateral 
motion of the lower jaws than would those of Castor. At the 
base of the auditory bulle, and in front of the mastoid of 
Castoroides, occurs a deep and rough pit for muscular attach- 
ment that is not found in Castor. 
On plate 27, figure B, a lateral view of the Boicourt speci- 
men is given. The large broken incisor is withdrawn from 
its socket and the thin inner wall removed, showing the depth 
of the socket, which reaches back to the roots of the first 
molar, a thin partition only separating the pulpy root of the 
incisor from that of the molar. Just above, at b and ¢, are 
seen the frontal and nasal sinuses. A portion of the inner 
wall of the maxilla has been removed to show the last molar 
(d) as it lies in its alveolus, while at e is shown the brain 
cavity. 
