66 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



groove or channel along each of its borders. It is from this resemblance to a 

 portion of a fluted column, that the animal takes its specific appellation (Meg\ 

 laqueatus). 



The crown would resemble an irregular ellipsis widest at the anterior por- 

 tion. The tooth consists of a central pillar of bone surrounded with enamel, the 

 former of a dead white, the latter of a ferruginous brown colour : the transverse 

 diameter is more than two-thirds less than its length, whilst that of Meg*. Jeffer- 

 sonii is only one-third less— the antero-posterior diameter is one-half its length in 

 the former, and two-thirds less in the latter. The proportions of this tooth are 

 consequently totally at variance with that of its kindred species." [Vide PI. 



XII. fig. 7, 3, 9.]* 



Dr. Harlan describes also two claws of the fore-foot, a radius, humerus, 

 scapula, one rib, an os calcis, a metacarpal bone, certain vertebrae, a femur, and 

 tibia, of the same Megahnyx ; these parts of the skeleton, together with the tooth, 

 which so fortunately served to establish the generic relationship of the species 

 with the Megahnyx of Jefferson and Cuvier, were discovered in Big-bone- cave, 



Tenessee, United States. 



Dr. Harlan does not enter into the question of the generic characters of 

 Megahnyx, but it would seem that he felt them to rest not entirely on dental 

 modifications, foT he observes that " a minute examination of the tooth and knee- 

 joint renders it not improbable, supposing the last named character to be peculiar 

 to it, that if the whole frame should hereafter be discovered, it may even claim a 

 generic distinction, in which case, either Aulaxodon, or Pleurodon, would not be 

 an inappropriate name."| 



There can be no doubt, as it appears to me, with respect to a fossil jaw 

 presenting teeth in the same number, and of the same general structure, as in the 

 Megatherium, and 



inguish Megatherium from Megahnyx, that the Palaeontologist has no 

 other choice than to refer it, either as Fischer has done with Megahnyx, to a distinct 

 species of the genus Megatherium, or to regard it as the type of a subgenus distinct 

 from both. With reference, however, to the Pleurodon of Dr. Harlan, after a de- 

 tailed comparison of the cast of the tooth on which that genus is mainly founded, 

 with the descriptions and figures of the tooth of the Megahnyx Jejfersonii, in the 

 " Ossemens Fossiles," they seem to differ in so slight a degree as to warrant only a 

 specific distinction, and this difference even, viewing the various proportions of 

 the teeth in the same jaw of the Megatherium, is more satisfactorily established 

 by the characters pointed out by Dr. Harlan in the form and proportions of the 

 radius, than by those in the tooth itself. 



* Medical and Physical Researches, pp. 323 — 4. 

 t Loc cit. p. 330. 



