420 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
the 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 curvature 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 molars differ strikingly in structure not only from those of Cas/or, 
‘but from those of all other Rodents except the Chinchillide, a near resem- 
blance being met with elsewhere only in the last molar of Hydrocherus. "They 
consist of a series of laminze of dentine completely inclosed by enamel, held 
together by a thin coating of cement. The circumference of the triturating 
surface of the tooth is thus devoid of the continuous plate of enamel that 
forms an uninterrupted border in the molar teeth of ordinary Rodents, and is 
deeply serrated. The dentinal laminze, with their inclosing plate of enamel, 
are three in number in all the molars except the last upper and first lower, 
which have each four. When the teeth are exposed to disintegrating influ- 
ences, the lamin of dentine and enamel readily fall apart, as is the case in 
the molars of the Chinchillida, in the last molar of Hydrocherus, and in the 
molars of the Elephant. In structure, the molar teeth of Castoroides are 
strictly comparable with those of the Chinchillide, and with the posterior 
portion of the last molar of Hydrocherus, and thereby differ not only radi- 
cally from that. seen in Castor, but from that of all other Rodents. The 
structure is precisely that seen in Lagostomus, even to the oblique position 
of the laminz, except that the number of the laminz in Castoroides is one 
more to each tooth (two more in the first lower molar) than in Lagostomus. 
The dentinal lamine are very similar also to the dentinal lamine of the last 
molar of Hydrocherus, but they are relatively much thicker. The molar 
teeth of Custoroides are thus compound, and have no resemblance to those of 
Castor, with which genus Castoroides is usually compared. 
In other features of the skull, strong resemblances can be traced between 
Castoroides and Lagostomus, especially in respect to the form of the pterygoid 
processes and the size and form of the pterygoid fosse.t The general form 
* Wyman, Bost. Journ. Nat. Hist., vol. v, pp. 394, 395. 
tIn these features, however, Fiber much more nearly approaches Castoroides than does Lagostomus. 
