RODENTI A CASTORINAE CASTOR CANADENSIS. 



357 



rare. The lightest specimens I have seen are 1548 from the upper Missouri, and 1337 from 

 the Colorado river of California. 



Measurements of No. 898. 



lii young specimens the tibia and fibula are distinct but anchylose with age. The line of junction is, however, quite 

 distinct, even in old specimens. 



Skull. The skull of the beaver is very massive and heavy, and has many peculiarities not 

 shared by other genera of rodents. The nasal bones are broad and short, widest anteriorly and 

 narrowing posteriorly ; the external border a convex curve. The line drawn across the pos 

 terior extremity of these bones falls either on the small tuberosity of the ante-orbital process of 

 the frontal or a little behind it, in no case reaching as far as the middle of the orbit, as in the 

 European beaver. The nasal processes of the intermaxillary do not extend quite so far back, 

 but attain a point between the tuberosities referred to. There is no post-orbital process to the 

 frontal bone, although a tubercle not quite so prominent as that in the anterior process may, 

 perhaps, be considered to represent one. The malar bone is much as in the Sciurinae, except 

 that it is proportionally much higher in its middle portion. The anterior border of the zygo- 

 matic arch is not formed by the malar bone, but by the maxillary which shows a thin edge 

 applied against the malar for half its extent exteriorly. The malar extends up to the anterior 

 end of the orbit, which is there constituted by it, and is separated from contact with the frontal 

 by the small subquadrate external face of the lachrymal. The malar sends up a broad flattened 

 angular process which constitutes the postero-inferior wall of the orbit. Posteriorly it passes 

 along the under surface of the malar process of the temporal, and passes a little behind this 

 process. It also forms the exterior wall of the glenoid cavity. 



The incisive foramina are long and narrow, situated about midway between the incisors and 

 molars, occupying rather less than one-third of this interval. They are situated almost entirely 

 in the intermaxillary bones ; the posterior ends, with a thin vertical plate, (extending forward 

 for half their length,) belonging to the maxillary. The palatine bones extend forward in an 



