FOSSIL MAMMALIA. 101 



forms the upper boundary of a pretty deep fossa, which is divided by a median 

 vertical ridge, extending downwards to within an inch of the upper margin of the 

 foramen magnum. A second strong obtuse transversely arched ridge curves 

 over the first, and forms the upper boundary of the posterior or occipital region 

 of the skull : the interspace between the two transverse ridges is very irregular, 

 and indicates the firm implantation of powerful nuchal muscles or ligaments, 

 (PI. XXX. fig. 1.) 



In the configuration and angle of the occipital plane the Megatherium indi- 

 cates the same general correspondence with the Edentate type, which has been 

 pointed out in the descriptions of the crania of the Glossothere and Scelido- 

 there : and the resemblance to the Scelidothere is not less striking in the small 

 proportional size of the cranium in this quadruped, which surpasses the rest of its 

 class in so great a degree in the colossal proportions of its hinder parts. 



Having detected in the base of the skull of the Scelidothere an articular 

 semicircular pit for the head of the styloglossal bone, similar to, but relatively 

 smaller than, that remarkable one in the skull of the Glossothere, it became a 

 matter of interest to determine whether this structure, which does not exist in any 

 of the existing Edentals, should likewise be present in the gigantic type of the 

 Megatherioid family. The result of a careful removal of the matrix from the 

 basal region of one of the cranial fragments of the Megatherium was the detec- 

 tion of this articular cavity, in each temporal bone in the same relative position 

 as in the Glossothere and Scelidothere. The styloid articular cavity is relatively 

 smaller, and shallower, than in the Glossothere, its proportions being much the 

 same as those of the Scelidothere. The cranial or posterior extremity of the 

 stylo-hyoid bone in the Scelidotherium is bent upwards at an obtuse angle (PI. 

 XXL), and terminates in an articular ball which rotates in this cavity. The size 

 of this bone, and its mode of articulation, indicates great power and muscularity 

 of tongue in the Megatherioids, and calls to mind the importance of that organ in 

 the Giraffe, which subsists on the same kind of food as that which I have sup- 

 posed to have supported the Megatherioids, although the general organization of 

 these animals and the mode in which the foliage was brought within reach of the 

 tongue are as opposite as can well be imagined. 



The anterior condyloid foramen presents scarcely one half the absolute 

 size of that of the Glossothere, whence we may infer a correspondingly 

 inferior development of the tongue in the Megathere. The fractured parietes 

 of the cranial cavity of the Megatherium every where exhibit evidences of 

 the great extent of the air-cells or sinuses continued from the nasal cavity : 

 on the basilar aspect of the cranium they extend as far back as the jugular 

 foramina : the whole of the basi-sphenoid being thus excavated, and permeable 



