FOSSIL MAMMALIA. 103 



forwards ; the fourth is straight, its anterior surface only describing a slight con- 

 vexity in the vertical direction ; the fifth tooth is curved, but in a contrary direc- 

 tion to the others ; and the bases of the five molars thus present a general con- 

 vergence towards a point a little way behind the middle of the series. 



The next peculiarity to be noticed in these remarkable teeth is the great length 

 of the pulp-cavity (d), the apex of which is parallel with the alveolar margin of the 

 jaw : a transverse fissure is continued from this apex to the middle concavity of the 

 working surface of the tooth, which is thus divided into two parts. Each of 

 these parts consists of three distinct substances, — a central part analogous to the 

 body or bone of the tooth or ' dentine,' a peripheral and nearly equally thick 

 layer of ccementum, and an intermediate thinner stratum of a denser substance, 

 which is described in Mr. Cliffs memoir on the Megatherium as 'enamel,' and to 

 which substance in the compound teeth of the Elephant, it is analogous both in 

 its relative situation, and relative density to the other constituents. 



Microscopic examinations of thin and transparent slices of the tooth of the 

 Megatherium prove, however, that the dense layer separating the internal sub- 

 stance from the csementum is not enamel, but presents the same structure as the 

 hard ' dentine' or ivory of the generalit}' of Mammalian teeth ; and corresponds 

 with the thin cylinder of hard ' dentine' in the tooth of the Sloth. No species 

 of the Order Bruta has true enamel 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 than that of the 

 enamelled teeth of the Herbivorous Pachyderms and Ruminantia, in consequence 

 of the introduction of a dental substance into their composition corresponding in 

 structure with that of the teeth of the Myliobates, Psammodus, and other cartila- 

 ginous fishes. 



The microscopic investigation of the structure of the teeth of the Megathe- 

 rium was undertaken chiefly with the view of comparing this structure with that of 

 the teeth of the Sloth and Armadillo, and of thus obtaining an insight into the 

 food, and an additional test of the real nature of the disputed affinities of the 

 Megatherium. The central part of the tooth (c. PI. XXXI.) consists of a coarse 

 ivory, like the corresponding part of the tooth of the Sloth. It is traversed 

 throughout by medullary canals T^ioth of an inch in diameter, which are con- 

 tinued from the pulp-cavity, and proceed, at an angle of 50°, to the plane of the 

 dense ivory, parallel to each other, with a slightly undulating course, having 

 regular interspaces, equal to one and a half diameters of their own arete, and 

 generally anastomosing in pairs by a loop of which the convexity is turned 

 towards the origin of the tubes of the fine dentine, as if each pair so joined con- 

 sisted of a continuous reflected canal, (c. fig. 1, PI. XXXII.) The loops are gene- 



