ANATOMY OF THE BEAVER. 67 



without undue pressure on their extremities. The 

 curves are rendered necessary also by the position of 

 the teeth in the jaws; the distance between the upper 

 molars, from side to side, being less than that of the 

 lower. The lower set are also longer antero-poste- 

 riorly by half the length of the crown of a tooth than 

 the upper. set. The cementum is found on the out- 

 side of the teeth and in the spaces where there are 

 inflections of enamel ; but where dentine is opposed to 

 dentine it is not deposited in layers; and, if at all, 

 only in a granular form. The question arises whether 

 the molars, like the incisors, belong to the rootless 

 class of teeth. In Prof. Baird's elaborate Report on 

 Mammals,^ the sub-family Castorinae, embracing the 

 genera Castor, Aplodontia, and Castoroides, is defined 

 as having "rootless molars." Brandt {op. cit., p. 301) 

 defines the family Castoroides— genus Castor — as hav- 

 ing "molares radicati" — rooted molars. If we exam- 

 ine the molars of the beaver in the young skull, in 

 their immature condition (Fig. 4), they are found to be 

 prismatic; their extremities in the jaws are expanded, 

 and present all the inflections of enamel 

 seen on the crown surface. In this, 

 their primitive condition, they grow as 

 do other rootless teeth, until the jaws 

 have attained their development. The 

 tooth then becomes rooted (Fig. 5) and 

 incapable of further growth — the pulp 



cavity contracts, the opening becomes 



•^ ^ r o less" molar 



Section of "root- 

 lateral, and is sometimes entirely closed; 



' General Report upon the Zoology of the several Pacific Rail- 

 road Routes, vol. viii. Mammals. By Spencer P. Baird. Wash- 

 ington, D. C, 1857. 



