Castoroides Ohioensis. 395 
occipital bone ; the second, inferior, is formed by the origins 
of the same processes and the posterior edges of the ossa 
palati. 
The pterygoid fossa has a depth of about two inches, which, 
added to the great breadth of the outer process and the curva- 
ture of the inner, gives an extraordinary surface for the origin 
of the internal pterygoid muscle. The fossa serving for the 
origin of the external pterygoid muscle, involves the whole of 
the greater wing of the os sphenoides, and is more remarkably 
developed than in any of the allied genera. 
The triangular-shaped palatine space comprised between 
the two ranges of molars has a length of two inches; posteri- 
orly it is 1.8 inch in breadth, but anteriorly is so much con- 
tracted as to leave a space of 0.3 inch only between the first 
molars, The posterior palatine foramina are elongated ellipti- 
cal openings, having a longitudinal diameter of 0.5 inch, and 
directed obliquely outwards. 
he ossa palati terminate anteriorly, opposite the space be- 
tween the first and second molars, at which point commences 
à ridge, at first not well defined, but afterwards becoming well 
Marked, and extending as far forwards as the foramen incisi- 
vum ; commencing in front of the first molar, on each side, is 
‘nother ridge, less distinct, and terminating on the side of 
1e same foramen. In the Castors the central ridge extends 
backwards quite to the posterior edge of the palatine bones. 
The incisive foramen, which in the Castors, Ondatras, Mar- 
mots, Agoutis, & c. acquires so great a size, is in the Casto- 
Toides Proportionally remarkably small, scarcely allowing the 
of an ordinary probe. : 
The anterior edge of the first molar is situated just in the 
middle of the base of the skull, but in the other genera above 
"eerred to, it is always in advance of the same point. The 
alveolar portion of the intermaxillary, situated just below the 
rasal orifice, presents a deeply indented and roughened sur- 
face, Serving for the attachment of the upper lip, doubtless 
"nüsua]ly developed in order to conceal the large incisor 
