1883.] The Extinct Rodentia of North America. 371 
chillidæ, a near resemblance being met with elsewhere only in the 
last molar of the Capybara. They consist of a series of laminæ 
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 lamin, with their 
iR 23.—Castoroides ohioensis Fost., two fifths nat. size. a, Incisive foramen ; 
Sihi pe fossa; c, Internal pterygoid plates; d, Fossa in basioccipital; e, Ex- 
dana J; Mastoid process; g, Condyles; 4, Tympanic bulla. After 
n. 
nclosing plate of enamel, are three in number in ali the molars, 
we the last upper and first lower, which have each four. 
lami the teeth are exposed to disintegrating influences, the 
in of dentine and enamel readily fall apart, as is the case in 
molars of the Chinchillidz, in the last molar of the Hydro- 
» and in the molars of the elephant. In structure the 
