138 



May 20, 1846. 



A. A. Gould, M. D., in the chair. 



Professor Jeffries Wyman read a report on the fossil cra- 

 nium and lower jaw of an extinct Rodent, which had been 

 referred to him for examination ; the same which is men- 

 tioned on page 103 of this volume. 



Professor Wyman considers it to have belonged to the same 

 animal, of which a lower jaw, and an incisor tooth of the upper 

 jaw, were described by Mr, J. W. Foster, one of the assistants in 

 the Geological Survey of Ohio, and to which the name of Cas- 

 toroides Ohioensis has been given. The subject of Professor 

 Wyman's remarks, and those described by Mr. Foster, are the 

 only remains of the animal which have been observed hitherto. 

 The cranium surpasses in size that of any fossil or existing ani- 

 mal, referable to the same order. 



On comparing it with other skulls of Rodents, it presents 

 some analogies to the genera Castor, Fiber, and Hydrochcerus. 

 Osteologically considered, it has stronger affinities with the Cas- 

 tors, than with either of the other genera ; but, in the dentition, 

 the type is totally different from that of the Castors, and not 

 unlike that of the HydrochcBrus ; to the conformation of the 

 pterygoid processes and fossae, some resemblance exists in the 

 Fibers. 



It also differs from the Castors in the much smaller relative 

 capacity of the cerebral cavity, in the greater depression of the 

 occiput, in the form of the condyles and of the foramen mag- 

 num ; the former admitting of a free motion vertically, but of a 

 very limited one in a lateral direction, and the latter having an 

 oval form and destitute of an emargination on its upper border ; 

 it differs from the Castors, also, in the size of the pterygoid pro- 

 cesses and fossaj, especially in the incurvation of the internal 

 ones, and the consequent subdivision of the posterior nares ; in 

 the compound nature of the molares, and in the fluting of the 

 anterior face of the incisive teeth ; the diminutive size of the 

 incisive foramina, and in the conformation of the lower jaw, with 



