44 



The Megatherium agrees, as has already heen shown, with the Mylodon, 

 SceUdotherium and Bradypus in the number, structure, and fangless condition of 

 the teeth. Its claims to generic distinction in regard to dental characters, rest, 

 therefore, on modifications of the same kind as those which have led to the for- 

 mation of the genera Megalonyx, Mylodon, and SceUdotherium, and which have 

 influenced naturalists in the formation of many acknowledged genera of existing 

 Mammalia. The differences which will be subsequently pointed out in the 

 locomotive organs of the Megatherium, Megalonyx, Mylodon, and SceUdotherium, 

 fuUy justify the confidence which may be placed in the dental characters on 

 which these genera were originaUy estabUshed. Those by which the Megathe- 

 rium is distinguished from the subsequently discovered extinct members of the 

 same natural family, are derived from the shape, and the greater proportion of 

 the cement or external constituent of the teeth. 



All the teeth of the Megatherium are larger in proportion to their thickness 

 than in the Mylodon, Megalonyx or SceUdothere. They are also more closely 

 approximated, and, as has been shown in the comparison of the skuUs, the in- 

 terval separating the right from the left molar series is narrower. The first tooth 

 in the upper jaw is close to the second, as in the SceUdothere and the three-toed 

 Sloth : but it presents a nearly semicircular transverse section, with the con- 

 vexity turned forwards and the angles rounded off : the three succeeding molars 

 are four-sided, with the transverse somewhat exceeding the antero-posterior di- 

 ameter, and with the outer and narrowest side slightly depressed : the last or 

 fifth molar is likewise four-sided, but is only half the size of the preceding, and 

 has a relatively greater transverse diameter. The first and fifth molars are 

 gently curved, with their concavities turned towards each other : the second and 

 tliird teeth are nearly straight ; the fourth is quite straight. To judge from the 

 figures by Bru and Pander of the Madrid Megatherium, the first, second and 

 third molar teeth of the lower jaw correspond with those above ; but the shape 

 of the fourth molar is not indicated in either their figures or descriptions. It 

 is obviously most desirable to learn whether it presents the large proportional 

 size and complex form which are its characteristics in the Mylodon and Scelido- 



terlor part of the astragalus, determine the Megalonyces Cuvieri and Bucklandi of Dr. Lund to be spe- 

 cies of the genus SceUdotherium. 



