FOSSIL MAMMALIA. 



99 



of the astragalus of the Scelidothere from the Megatherioid to the Dasypodoid 

 type of structure. For if the Megatherioid type of structure had really been one 

 suitable to the exigencies of climbing quadrupeds, it might have been expected to 

 have exhibited the scansorial modifications more decidedly, as the species 

 diminished in stature ; but as regards the instructive bone of the hind-foot, 

 the modifications of which we have just been considering, this is by no means 

 the case. 



DESCRIPTION OF A MUTILATED LOWER JAW OF THE 



MEGALONYX JEFFERSONII. 



In the preceding section an astralagus was described, which was regarded as 

 belonging possibly to the same Edentate species as the jaw figured and described, 

 p. 69, PI. XVIII. and XIX., under the name of Mylodon Darwinii ; but the same 

 correspondence, — that of relative size, — renders it equally possible that this 

 astragalus may belong to the species of Megalonyx to which the lower jaw now 

 under consideration appertains. There could be no doubt, from its structure, 

 that it was the astragalus of a gigantic species of the order Bruta, and of the 

 Megatherioid family, and more nearly allied to the Megathere than is the Scelido- 

 there, but sufficiently distinct from both. 



The lower jaw, figured in PI. XXIX., is the only fossil brought home by 

 Mr. Darwin that could be confidently referred to the genus Megalonyx ; but the 

 form of the tooth in place on the right side of the jaw fully justifies this deter- 

 mination. The jaw itself is deeply and firmly imbedded in the matrix, so that 

 only the upper or alveolar border is visible. The coronoid and condyloid pro- 

 cesses are broken away, and the texture of the remaining part of the jaw was 

 too friable, and adhered too firmly to the surrounding matrix to admit of more of 

 its form being ascertained than is figured. 



There were four molars on each side of this jaw ; the large oblique perfora- 

 tion near the fractured symphysis is the anterior extremity of the wide dental 

 canal. The forms of the alveoli are best preserved in the right ramus: the first 

 is the smallest, and seems to have contained a tooth, of which the transverse 

 section must have been simply elliptical : the second tooth is likewise laterally 

 compressed, but the transverse section is ovate, the great end being turned 

 forwards : the third socket presents a corresponding form, but a larger size : the 

 fourth socket is too much mutilated to allow of a correct opinion being formed as 

 to the shape of the tooth which it once contained. The natural size of the tooth 



o 2 



