A Skeleton of Great Fossil Beaver, Castorotdes Ohtoensis. 149 
The atlas, while it bears a general resemblance to that of Castor, 
is, in some important respects, different. The lateral diameter, 
including processes, is 3.62 inches; from tubercle to extremity of 
spinous process, 1.75 inches; across anterior articular surfaces, 
2.37 inches; posterior articular surfaces, extreme diameter, 2.12 
inches; vertical diameter of neural arch, 1.12 inches; lateral diam- 
eter of neural arch, 1.06 inches. 
The area below the transverse ligament appears to be relatively 
greater than in Castor, and that above the ligament relatively less. 
The anterior articulating surfaces, which articulated with the 
occipital condyles, show that there was less of the ball and socket 
character than in Castor, as their transverse areas are relatively 
greater, and their depth relatively less. 
The neural groove above and anterior to the posterior articulating 
surfaces is deep and very narrow, so much so that it is almost a 
neural foramen. ‘The lateral processes are much less flattened and 
wing-like, less bent upward and backward, and the spinous process 
is more developed than in Castor. ‘The foramina are indicated by 
the straws in the illustration, and for number, position and direc- 
tion. correspond very nearly with what one may see in the atlas 
of Castor. ‘The spinous process is about one-fourth of an inch 
long, with an anterior groove and perceptibly double at the 
extremity. 
The axis and the third cervical are so thoroughly anchylosed 
as to be one solid piece. This double character was not dis- 
covered till an effort was made to homologize the furamina and 
other parts with the same in other rodents, but the neural foramina 
at the sides, the double character of the transverse processes 
and various other marks make its character very clear. ‘lhe neural 
arch of this double vertebra is nearly semicircular, being one 
inch transversely at the base, and half an inch vertically. From 
lower surface of centrum to tip of spine is 2.12 inches. Between 
extremes of transverse processes, 2.85 inches. 
In Castoroides the transverse diameter of the axis is almost equal to 
that of the atlas, while in Cus¢or it decreases to little more than half 
the lateral extent. The lateral processes are bent downward, as 
well as backward. ‘The anterior articulating surfaces are percep- 
tibly convex, and are laterally .85 of an inch, by .75 vertically. 
They slope backward at an angle of about forty-five degrees. 
The third cervical (fused to the axis) measures 2.12 inches 
