294: DENTAL SYSTEM OF THE MEGATIIEKIUM. 



acquire the requisite room for the lodgement of the lower 

 teeth and their " matrices," or formative organs. 



The next peculiarity to be noticed in these remarkable 

 teeth is the great length of the conical cavity at their 

 base, for lodging the part of the matrix called the " pulp ;" 

 the apex of the pulp-cavity rising as far as the part of the 

 tooth where it emerges from the socket. A transverse 

 fissure is continued from this apex to the middle concavity 

 of the grinding surface of the tooth, which is thus divided 

 into two halves. Each of these halves consists of three 

 distinct substances — a central column of " vaso-dentine," 

 a peripheral and nearly equally thick layer of " cement," 

 and an intermediate thinner stratum of true or "hard- 

 dentine." This latter has been described as being enamel ; 

 but it is only analogous to that differently constituted 

 and harder substance in the compound teeth of the ele- 

 phant, in regard to its relative situation, and its degree of 

 density to the other constituents of the tooth of the me- 

 gatherium. 



No species of the order called "Bruta," or "Edentata," 

 to which the extinct megatherium belongs, has true ena- 

 mel entering into the composition of its teeth; but the 

 modifications of structure which the teeth present in the 

 different genera of this order are considerable, and their 

 complexity is not less that that of the enamelled teeth of 

 the herbivorous, ruminant, and other hoofed animals, in 

 consequence of the introduction of a dental substance — the 

 " vaso-dentine" — into their composition, analogous in struc- 

 ture to that of the teeth of the Myliohates and other carti- 

 laginous fishes. The cement of the megatherium's tooth 

 differs from the vaso-dentine in the larger size and wider 

 interspaces of its medullary canals, and by the presence 

 of radiated bone-cells in their interspaces ; but they are 



