41 



Comparison of the Teeth. 



The structureof tooth above described is pecuhar, among existing Mammalia, 

 to the Sloths ; and is doubtless especially adapted for the comminution of the 

 foliage which constitutes their food*. The number of the teeth, their length 

 and absence of fangs, their excavated base, and the unlimited growth resulting 

 from the persistent pulp, together with their composition of cement, unvascular 

 and vascular dentine, are characters common to the Megatherioids and Sloths. 

 The form of the teeth, which determines that of their grinding surface, differs 

 in, and is characteristic of, the different genera. It would seem that the Mega- 

 lonyx, in the simple elliptical or subcyhndrical shape of such of its teeth as are 

 known, more closely resembled the existing Sloths than do the other Megathe- 

 rioids. If the tooth of the Megalonyx possessed, as has been affirmedf, true 

 enamel, it would materially differ not only from those of the Mylodon and Sloths, 

 but from every other Edentate animal. I have, however, ascertained by ex- 

 amination of a tooth in the lower jaw above mentioned, that the hard part which 

 Cuvier thought to be enamel, is the unvascular dentine or ivory ; and that, in 

 the large proportion of the vascular dentine in the centre and of the cement at 

 the circumference, the teeth of the Megalonyx correspond with those of the 

 Sloths, not with those of the Armadillos^. 



The Unau resembles the Mylodon in the separation of the first from the other 

 upper molars, but the detached tooth assumes, in the existing Sloth, the form and 

 proportions, as well as the position, of a canine ; and the corresponding tooth of 

 the lower jaw is similarly developed and separated from the other three teeth by 

 a nearly equal interval. In the adult Ai the first molar of both jaws presents 



* See the magnified section of a molar of the Bradypus tridactylus, Plate XXIV. fig. 1. 



Cecropia peltata and Achra sapota have been particularized as affording nutriment to the Sloths, but 

 few of the trees forming the denser forests of tropical America are exempt from their attacks. 



"t" Cuvier, describing the tooth of the Megalonyx, says, " Je I'avois cnie d'abord necessairement de 

 Paxesseux ; mais aujourd'hui que je connais mieux I'osteologie des divers Tatous, je trouve qu elle res- 

 semble au moins autant a une dent de I'un des grands Tatous. Dans ces deux genres, les dents sont 

 de simples cylindres de substance osseuse enveloppes d'un etui de substance emailleuse." — Ossemens 

 Fossiles, ed. cit. torn. viii. p. 329. 



{ Zoology of the Beagle, ' Fossil Mammalia,' 4to, p. 69. 



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