598 



OBSERVATIONS ON THE RODENTIA. 



the upper jaw, and not as represented in Cuvier's plate in the 

 Ossemens Fossiles, vol v. pt. 1, pi. 3. I mention this because 

 being myself in the habit of twisting the jaw of these rodents in 

 an unnatural position when comparing them with others, it was 

 some time before I perceived its peculiar characters, charac- 

 ters which are important, inasmuch as they are combined 

 with numerous others. 



Skull and lower jaw of Castor Fiber. 



Upon comparing the jaw of the beaver [Castor Fiber) with 

 that of other rodents, the most striking characters consist in 

 the large size of the coronoid process and the form and posi- 

 tion of the descending ramus, or that part (* in the figures) 

 which lies below and behind the alveolus of the inferior inci- 

 sor. In the species of rodents belonging to the several fami- 

 lies already pointed out, the descending ramus approaches 

 more or less to a quadrate form, the upper posterior angle 

 being generally acute, but the lower part more or less round- 

 ed. The descending ramus of the beaver differs in being 

 more extended in the direction parallel with the dental por- 

 tion of the jaw, and less extended in a transverse direction; 

 and if the jaw be placed in its natural position the angle (*) 

 is situated above the plane of the symphysis menti, or above 



