r ¥ ~ °, *® r te a 
oa 
, 
148 Cincinnats Society of Natural History. 
projecting inward and backward from the postero-interior base of 
the ramus. Another notable feature is a large external fossa 
below the sigmoid notch for the insertion of the masseter muscle. 
The depth of this fossa is largely due to the swelling outward, 
immediately below it, of the lower part of the ramus in that 
region, to accommodate the base of the deeply-planted incisor. 
The coronoid process in Castor stands well above the nearly 
sessile condyle, making the sigmoid notch comparatively shallower. 
Said process is relatively greater than in Castoroides, but not so 
much so as might at first appear, since the relatively greater length 
of the condyle in Castoroides is so marked as to make the process 
comparatively low. ‘The condyle, with its neck, is more than 1.5 
inches long, measuring from the level of the bottom of the sigmoid 
notch. ‘The condyle has a wide and well-marked groove poste- F 
riorly, which is barely perceptible in Castor. Owing to the bend- ! 
ing inward of the ascending rami and the necks of the condyles, 
the inner faces of the latter look toward each other (as well as 
upward) and stand less than 2.5 inches apart, which is relatively 
much less than in Casfor, and indicates how narrow was the brain 
cavity for so large an animal. 
The mental foramina are immediately below the anterior molars, | 
but quite above the upper line of the imbedded incisors. Nearly 
an inch backward from the posterior molars, and in the base of the 
ascending ramus, is the dental foramen, of the diameter of a rye 
straw. Below this, along the deepest groove of the pterygoid 
fossa, is a line of half a dozen or more small foramina which prob- 
ably lead to the pulp cavity of the posterior incisor at its base. 

Fie. 9.—One-third natural size. a, anterior and upper view of atlas: straws indicate 
the foramina and their directions; 6. anterior view of axis. 

