43 



minence from one side, their specific distinction, at least, from the above-cited 

 species of Mylodon. The Darwinian specimen of Megalonyx more satisfactorily 

 establishes the distinction by showing that both the second and third lower 

 molars present the elHptical section characteristic of the teeth of Megalonyx, 

 and have no trace of the longitudinal channels which distinguish the corre- 

 sponding teeth in the genus Mylodon ; and the Brazihan species of Scelidothe- 

 rium, described under the name of Megalonyx Cuvieri by Dr. Lund*. 



In the Scelidotherium leptocephalumf the dentition of both the upper and lower 

 jaws can be compared with that of the Mylodon. The first molar is not divided 

 by a disproportionate interspace from the rest : its transverse section gives a 

 narrow inequilateral triangle, with rounded angles, and the base turned inwards 

 and obliquely forwards. The second molar also, instead of an eUiptical trans- 

 verse section, presents also a triangular one with the angles rounded off, and 

 two of the sides slightly indented : it resembles the antepenultimate molar in 

 the Mylodon robustus. The third and fourth molars of the Scehdothere are more 

 compressed than in the Mylodon ; the long axis of their transverse section is 

 from before backwards, instead of transversely. The fifth molar has a trihedral 

 form, with the broadest side turned outwards and slightly indented. In the 

 lower jaw of the Scehdothere the differences in the form of the teeth are equally 

 manifest, especially in the prismatic form of the first molar : the last molar re- 

 sembles that of the Mylodon Darwinii, the grinding surface of this tooth being 

 divided into two lobes by two oblique channels, which traverse longitudinally, 

 one the outer, the other the inner side of the tooth ; but these are shallower 

 than in the Mylodon Darwinii, and the lobes are more equal and more flattened. 

 The two middle teeth differ more markedly from the corresponding ones in any 

 of the species of Mylodon : the transverse section of both these teeth presents, 

 in the Scehdothere, a compressed oval form, with the large end turned obhquely 

 forwards toward the outer side of the jaw, and slightly indented, whilst the 

 inner and narrower end of the section is convex. The first lower molar in the 

 Scehdothere corresponds in form with the first upper molar, and is consequently 

 easily distinguishable from the corresponding tooth in the genus Mylodon}. 



* Loc. cit., tab. 3 to 9. t Fossil Mammalia of the Beagle, p. 81. pi. 23. 



t TTie form of the first and last molars of the lower jaw, and the two concave surfaces upon the an- 



f2 



