45 



therium ; and whether it thus equals the two opposing molars above, or whether 

 these be opposed by two corresponding molars of unequal size below. 



The grinding surface of the close-set molars of the Megatherium differ, on ac- 

 count of the greater thickness of cement on their anterior and posterior surfaces, 

 from those of all the smaller Megatherioids in presenting two transverse ridges ; 

 one of the sloping sides of each ridge being formed by the cement in question, 

 the other by the vascular dentine, whilst the hard unvascular dentine forms the 

 summit of the ridge. These modifications, with the narrow palate, the close-set 

 series of teeth, their great length, and the concomitant depth of the jaws, are 

 features of resemblance to the maxillary and dental characters of the Elephant ; 

 but the fundamental structure and nature of the teeth, not only of the Megathe- 

 rium, but of all the allied extinct species, are manifested in the present day ex- 

 clusively by the restricted and diminutive family of the Sloths. These mammals 

 present to the zoologist, conversant only with living species, a singular exception 

 in their dental characters to the rest of their class ; but there has been a time 

 when this pecuhar dentition was manifested under as various modifications as 

 may now be traced in some of the more common dental types in existing orders 

 of Mammaha. 



Description of the Os Hyoides. 



A symmetrical bony arch, measuring eight inches along its convex side, with 

 two articular tubercles defining the limits of the body and crura, the expanded 

 extremities of which support a small flat articular surface, evidently represents 

 the body and posterior cornua [cornua majora in Anthropotomy) of the os 

 hyoides. The part of the body included between the articular tubercles for the 

 anterior cornua is short, subcyUndrical, and of less vertical diameter than the 

 anchylosed posterior cornua. A rather thick and rough convex ridge projects 

 downward from the anterior extremities of these processes, doubtless for the at- 

 tachment of strong thyreo-hyoidei : two slight tuberosities on the front of the 

 body, below the articular tubercles, indicate the insertions of the sterno-hyoidei. 

 The under part of the body is not flattened nor expanded. No parts corre- 

 sponding with the stylo-hyal or cerato-hyal elements, which together form the 

 long anterior cornua {cornua minora in Anthropotomy) in the Sloths, could be 



