DENTAL SYSTEM OF THE MEGATHERIUM. 295 



brouglit into organic communication with each other, not 

 only by means of the tubes of coarse dentine, but by 

 occasional continuity of the vascular canals across that 

 substance. The tooth of the megatherium thus offers an 

 nnequivocal example of a course of nutriment from the 

 dentine to the cement, and reciprocally ; so that the main 

 substance, or body of the tooth, can obtain the requisite 

 supply for its languid vitality from the vessels of the cap- 

 sale as well as from those of the pulp. 



The conical cavity at the base of the tooth attests the 

 large size, and demonstrates the form of the persistent 

 pulp in the living megatherium; the diameter of its base 

 -is equal to the part of the tooth Avhich is formed by the 

 combined dentine and vaso-dentine. From the gradual 

 thinning off and final disappearance of those substances 

 as they reach the base of the tooth, it may be inferred 

 that both were formed at the expense of the pulp. The 

 fine dentinal tubes must have been established and cal- 

 cified in the peripheral layer of the pnlp, which layer 

 must have been wholly so converted into the dentine ; 

 but as the deposition of the hardening salts proceeded 

 in the rest of the pulp, certain tracts of that soft and 

 vascular substance were left uncalcified, to form the me- 

 dullary or vascular canals which characterize the vaso- 

 dentine. The space between the inserted base of the 

 tooth and the walls of the socket indicates the thick- 

 ness of the dental capsule, by the ossification of which the 

 exterior layer of cement was formed ; and this modifica- 

 tion of the tooth-forming organ in the megatherium per- 

 mitted the progressive addition of cement, as the persist- 

 ence of the compound pulp occasioned the "uninterrupted 

 and continuous formation of the harder dentine, which is 

 analogous to the enamel in the elephant's grinder. 



