100 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



in situ, and of the adjoining socket, is given in PI. XXIX., fig. 2. The difference 

 of form which the jaw of the Megalonyx presents, as compared with that of the 

 Mylodon, especially in the greater recedence of the two horizontal rami from each 

 other, will be appreciated by comparing PL XVIII. with PI. XXIX. 





DESCRIPTION OF A FRAGMENT OF THE SKULL AND OF THE TEETH OF THE 



■ 



MEGATHERIUM CUVIERL 



Notwithstanding the full, accurate, and elaborate accounts of the skeleton of 

 the Megatherium given by Bru,* Cuvier,| Pander and D'Alton,J and Mr. Clift,§ 

 the fragments of this most gigantic of quadrupeds brought home by Mr. Darwin, 

 possess much interest, and have added, what could hardly have been antici- 

 pated, important information as to the dental system, whereby an error in the 

 generic character of the Megatherium has been corrected. 



The fragments here alluded to are portions of the skull of three full-grown 

 Megatheres : the most perfect part of which affords a view of the posterior, and 

 of part of the basal surface, which regions of the cranium have not hitherto been 

 elsewhere figured or described, (PI. XXX.) 



The plane of the occipital foramen forms with that of the base of the skull 

 an angle of 140°, the plane of the posterior surface of the skull forms with the 

 basal plane an angle of 68°. The occipital condyles are therefore terminal, or 

 form the most posterior parts of the cranium. The extent of their convex curva- 

 ture in the antero-posterior direction, which equals that of a semicircle, indicates 

 that the Megatherium possessed considerable freedom and extent of motion of 

 the head. The condyles are not extended in the lateral direction so far as in the 

 Toxodon; their axis is more oblique than in the Glossotherium, and their internal 

 surface is more parallel with the axis of the skull, the foramen magnum not pre- 

 senting that infundibuliform expansion which is so characteristic of the Glosso- 

 therium. The occipital condyles resemble most in form and position those of the 

 Scelidotherium ; but in the angle of the occipital plane the Megatherium is inter- 



mediate between the Scelidothere and Glossothere. The ex-occipitals terminate 

 laterally and inferiorly, each in a short, but strong obtuse process. The posterior 

 plane of the skull is traversed by a strong arched intermuscular crest, which 



* Descripcion del Esqueleto de un quadrupedo muy corpulento y raro, que se conserva en el Real Gabinete 

 de Historia Natural de Madrid. Folio, Madrid, 1796. 

 t Ossemens Fossiles, torn. v. pt. i. p. 179. 



% Das Riesen Faulthier, BradypuB giganteus, von Dr. Chr. Pander und Dr. E. D' Alton." Folio, Bonn, 1821. 

 § Transactions of the Geological Society, 1835, p. 438. 





