420 MONOGRAPHS OF NORTH AMERICAN EODENTIA. 



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 lias a depth of aboul 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 

 tiissa 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 Castor, 

 but from those of all other Rodents except the Chinchillidce, a near resem- 

 blance being met with elsewhere only in the last molar of llijdrochazrus. They 

 consist of a series of laminae 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 laminae, 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 laminae of dentine and enamel readily fall apart, as is the case in 

 the molars of the Chinchillidce, in the last molar of Hydrochcerus, and in the 

 molars of the Elephant. In structure, the molar teeth of Castoroides are 

 strictly comparable with those of the Chinchillidce, and with the posterior 

 portion of the last molar of Hydrocltcerus, 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 laminae, except that the number of the laminae in Castoroides is one 

 more to each tooth (two more in the first lower molar) than in Lagostomus. 

 The dentinal laminae are very similar also to the dentinal laminae of the last 

 molar of Hydrochcerus, but they are relatively much thicker. The molar 

 teeth of Castoroides 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 fossae.f The general form 



* Wymau, Bust. Jonru. Nat. Hist., vol. v, pp. 394, :>'J5. 



tlu Uiese features, however, I'ihcr much more nearly approaches Castoroides than does Layoslomus. 



