389 



cerned beneath the socket in both jaws. The deciduous molar is larger in the preceding 

 specimen of the British Beaver, which also shows the germ of a premolar above the socket of 

 the left upper deciduous molar. 



Hunterian. 



2164. A mutilated skull of the Canada Beaver (Castor canadensis), showing the 

 permanent dentition in situ. 



The premolar is larger than any of the true molars in the upper jaw, and its crown has a 

 greater antero-posterior extent than that in the lower jaw. 



Hunterian. 



2165. The cranium of a Canada Beaver (Castor canadensis), in longitudinal section, 

 with the implanted parts of the teeth exposed from the inner side in the right 

 moiety. 



The cerebellar depression of the petrosal is large and very deep. The anterior surface of 

 the petrosal is concave. The sella turcica is extremely shallow, and without clinoid pro- 

 cesses : the middle of the basioccipital is reduced by the excavation on its under surface to 

 extreme thinness. The small vacuity in the basisphenoid is exposed, which communicates 

 with the cranial cavity close to the ' fissura lacera anterior.' The presphenoid is perforated 

 transversely. The rhinencephalic fossa is well marked. The anterior end of the vomer arti- 

 culates with both the maxillary and premaxillary bones. 



Hunterian. 



2166. The cranium of a Canada Beaver (Castor canadensis), in three transverse 

 sections. 



The epencephah'c compartment is lower and broader than in the Porcupine. The cere- 

 bellar fossa of the petrosal is much larger and deeper. The upper compartment of the tym- 

 panum is much less. The length and direction of the auditory meatus, and its change of 

 form, into a transverse fissure, as it approaches the membrana tympani, the plane of which is 

 almost parallel with that of the meatus itself, are well shown in this section, as also the thin- 

 ning of the basioccipital produced by the depression on its under surface. The anterior sec- 

 tion includes the whole of the rhinencephalic fossa, and shows the transverse perforation of 

 the orbitosphenoids beneath the chiasmal platform. There are no nasal air-sinuses in the 

 cranial bones of this aquatic Rodent, and their texture is denser than in most of the Order. 



Presented by Prof. Owen, F.R.S. 



2167. The lower jaw of a Canada Beaver (Castor canadensis}, with the permanent 

 dentition. Hunterian. 



2168. The lower jaw of a Canada Beaver, with the permanent dentition. 



These duplicate specimens serve to show the slight amount of variation in the characteristic 

 pattern of the grinding surface to which the molar teeth are subject. 



Hunterian. 



