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Fig. 4. A slice of a tooth of a Megatherium, as viewed with a power of 

 500 linear dimensions. The teeth in this animal are somposed, as in 

 the Sloth and Mylodon, of a central axis of vascular dentine a, sur- 

 rounded by a layer of unvascular dentine b, and a coating of cement c ; 

 and differ principally in the great relative thickness of that outer co- 

 vering, which equals half the thickness of the vascular dentine, and 

 is six times thicker on the anterior and posterior sides of the tooth 

 than the layer of unvascular dentine. The vascular canals which tra- 

 verse the cement are more numerous than in the Mylodon, and 

 after branching dichotomously in their course from the outer to 

 the inner surface of the cement, terminate at the inner surface by 

 anastomosing in loops the convexity of which is turned towards the 

 hard dentine. The vascular system of the cement is thus very similar 

 to that of the vascular dentine, but the cement is distinguished by the 

 presence of the radiated calcigerous cells, which in number and ar- 

 rangement resemble those of the cement in the Mylodon and Sloth, 

 but have a less elongated elliptic figure. The vascular dentine in the 

 Megatherium closely corresponds in intimate structure with that of the 

 Mylodon. The fine calcigerous tubes of the unvascular dentine more 

 frequently bifurcate in their course towards the cement, but the artist 

 has rather exaggerated this character in the figure. The teeth of the 

 Scelidotherium and Megalonyx, which I have examined microscopi- 

 cally, agree in all their essential characters with those of the Sloth, 

 Megatherium and Mylodon, and most nearly resemble in their minor 

 modifications those of the latter genus, but have a greater proportion 

 of hard dentine. The large central axis of vascular dentine, and the 

 thick peripheral layer of cement, with the thin intervening layer of 

 hard or ordinary dentine, are peculiar to the phyllophagous family of 

 the order Bruta. 



The modifications of this characteristic dental structure which the extinct 

 members of the family have brought to light, give the plainest and most 

 striking demonstration of the vital activity which once pervaded every 



