Castoroides Ohioensis. 393 
Until the present. time no description of an entire cranium 
has been published, and as far as can be learned, the present 
is the first instance of the discovery of one in a perfect con- 
dition. It is that of an adult, measures 10.5 inches in length, 
and 7.2 inches across the broadest part of the zygomatic 
arches; the transverse diameter of the occiput is 5.5 inches, 
and that of the narrowest part of the cranium, between the 
orbits, is 1.9 inch. In its general outline it resembles that 
of the Castors ; but in its dentition it more closely resembles 
the Capybaras than any other Rodents, and among the Pachy- 
dermata, it presents close analogies to the elephants. To the 
Structure of the pterygoid fosse, analogies are found in the 
ndatras. 
. On comparing the cerebral portion of the skull with that 
Which lodges the nasal cavities, the former is relatively much 
smaller than in the Castors, Ondatras, and Capybaras. In 
Castoroides the longitudinal diameter of the cerebral cavity is 
less than two-fifths of the entire length of the skull; in the 
astor the same cavity is one-half, and in the Ondatra more 
than half the length of the skull. 
The upper surface of the cranium is much more flattened 
than in the Castors, and the interparietal crest extends the 
Whole length of the sagittal suture — posteriorly this crest has 
an elevation of nearly half an inch, where it is crossed by an- 
other, which separates the occipital from the coronal region ; 
anteriorly it terminates in two diverging ridges, which are lost 
on the upper edges of the orbits. A triquetrous bone rela- 
tively much smaller than in the Castor, exists at the union of 
the sagittal and lambdoidal sutures. 
The Occiput resembles that of the Castors but is more de- 
Pressed, having a transverse diameter of 5.5 inches, and a ver- 
tical one of 2.6 inches; its plane inclines forwards so as to 
an angle of about 80° with the base of the skull. The 
ramen magnum is very regularly oval, like that of Arctomy 5, 
!5 transverse diameter measuring 1.2 inches, and the vertical 
VOL. y, 26 
