A Skeleton of Great Fossil Beaver, Castoroides Ohuoensis. 147 
interest and convenience to such as may not have easy access to 
more full and complete descriptions. 
This rodent’s lower jaw is wider than long, leaving out the 
incisors in the measurement; its antero-posterior extent being 
7.55 inches, and its lateral 8.40 inches. ‘The greatest width is an 
inch or more forward of the posterior extremity of the incurved 
angles. The rami being disconnected, the large area of the sym- 
physis, with its ribbed and tuberculated surfaces, is clearly shown. 
Placing the left ramus beside the corresponding part of a well- 
developed Castor fiber, the process at the lower posterior point of 
the symphysis in the one, is very similar to that in the other, and 
about equally prominent in proportion to size. 
Holding each ramus in a horizontal position, teeth upward, the 
posterior third in Castor bends upward and gently inward, and 
from the angle a posterior and upper border reaches forward 
directly to the neck of the condyle. In Castoroides the posterior 
(less than) half of the ramus bends downward, abruptly outward, 
then sharply inward, terminating in the incurved twisted angle ; 
while between this prominent angle and the neck of the condyle is 
a notch in the posterior border 2.5 inches wide by one inch deep. 

Fie. 8.—One-third natural size. Upper and inner view of right ramus. Note 
grooves on the proximal and lingual sides of incisor, and worn inner surface. 
a, Angle showing inward bend; ¢, the S-shaped plate with fossa for pterygoid 
musele above; g, groove in posterior side of condyle and neck; s, symphysis. 
Probably the most extraordinary feature of this bone is the 
internal pterygoid fossa, which is well shown in the illustration, 
bounded at the bottom by an S-shaped blade .75 of an inch wide, 
