FOSSIL MAMMALIA. 105 



site humours after the central pulp has almost ceased to exist." In the Megathe- 

 rium, however, those anastomoses have not to perform a vicarious office, since the 

 pulp maintains its full size and functional activity during the whole period of the 

 animal's existence. It relates to the higher organized condition, and greater degree 

 of vitality of the entire grinder in that extinct species. 



The conical cavities (d. PI. XXXI.) attest the size and form of the persistent 

 pulp ; the diameter of its base is equal to the part of the crown of the tooth 

 which is formed by the coarse and fine dentine. From the gradual thinning off, 

 and final disappearance of these substances as they reach the base of the tooth, 

 I conclude that they were both formed at the expense of the pulp. The fine tubes 

 and cells must have been excavated in its peripheral layer for the reception of the 

 hardening salts of the dense dentine, and the rest converted into the parallel series 

 of medullary canals with their respective systems of calcigerous tubes, in a manner 

 closely analogous to the development of the entire tooth of the Orycteropus. 

 The coarser dentine of the tooth of the Megatherium differs, in fact, from the 

 entire tooth of the Orycteropus, only in that the parallel medullary canals and their 

 radiating calcigerous tubes are not separated from the contiguous canals by a 

 distinct layer of csementum, and that the medullary canals anastomose at their 

 peripheral extremities. The wide spaces, (e. PI. XXXI.) indicate the thickness 

 of the dental capsule by the ossification of which the exterior stratum of cement was 

 formed. It was not until I knew the true structure of the tooth of the Megathe- 

 rium, that I could comprehend the mode of its formation. The parallel layers of 

 enamel in the Elephant's grinder are formed, as is well known, by membranous 

 plates passing from the coronal end of the closed capsule towards the base of the 

 tooth ; but a certain extent of enamel can only thus be formed, and when the 

 crown of the grinder has once protruded, and come into use, the enamel cannot 

 be added to. The modification of the structure of the tooth of the Megatherium 

 readily permits the uninterrupted and continuous formation of the dense sub- 

 stance which is analogous to the enamel of the Elephant's grinder. 



With respect to the question of the respective affinities of the Megatherium 

 to the Bradypodoid or Dasypodoid families, the result of this examination of the 

 teeth speaks strongly for its closer relationship with the former group: the 

 Megalonyx, Mylodon, and Scelidotherium, in like manner correspond in the 

 structure of their teeth with the Sloth, and differ from the Armadillo. 



If from a similarity of dental structure we may predicate a similarity of food, 

 it may reasonably be conjectured that the leaves and soft succulent sprouts of 

 trees may have been the staple diet of the Megatherioid quadrupeds, as of the 

 existing Sloths. Their enormous claws, I conclude, from the fossorial character 

 of the powerful mechanism by which they were worked, to have been employed, 



p 



