141 



the absence of the dentes primores, or incisors, and of the requisite disposition 

 and flexibility of the fingers in the Megatherioid quadrupeds, negatives the con- 

 jecture that their clavicles bore an exclusive relation to such an action. 



Clavicles, though more commonly present in burrowing than in climbing 

 mammals, are by no means indispensable adjuncts to the fore-limbs in excavating 

 the soil. The badger, for example, is destitute of clavicles ; and these bones are 

 incomplete in both the rabbit and the fox. 



None of the feline animals, although they possess the requisite freedom of la- 

 teral and rotatory motion of the fore-extremities for striking and seizing their 

 prey, have more than mere rudiments of clavicles. 



It appears, then, that these bones are fully developed in reference to giving 

 due strength and stability to the shoulder-joint, for actions of the fore-arm not 

 necessarily employed in climbing, burrowing, or seizing a living prey ; and that, 

 when they are present in very excellent climbers, as the Unau and Orang, or in 

 the strictest and swiftest burrowers, as the Moles and Armadillos, they are asso- 

 ciated with modifications of other parts more immediately determining the pecu- 

 liar powers of such species. It must be, therefore, by a critical comparison of 

 the rest of the skeleton of the Mylodon that we can alone hope to determine the 

 nature of the actions in which its strong and perfect clavicles co-operated. 



A complete development of both bones of the fore-arm coexists, with free ro- 

 tation of the hand, in both digging and climbing quadrupeds ; but the climbers 

 which the Mylodon most resembles in the construction of the fore-feet are the 

 Sloths, and these are as remarkable for the length and slenderness of the arm 

 and fore-arm, as is the Mylodon for the shortness and robustness of the same seg- 

 ments of the anterior members. Not that the Mylodon equals in these characters 

 the true fossorial quadrupeds, such as the Mole ; but it thereby approximates 

 them too closely to admit of its being regarded as a mere climbing animal. 



To ascertain, on the other hand, whether the claviculate and strong arm, and 

 the freely rotating fore-arm of the Mylodon were used exclusively, when not en- 

 gaged in ordinary locomotion, for turning up the soil, it will be requisite to con- 



of the Marquis de Montmirail, thus describes this action : — " L'Unau saisit avec le pied de devant 

 comme avec une main, et s'en sert pour porter ses alimens a sa bouche;" — " ranimal en approchant 

 de son poignet I'extremit^ de ses ongles serre les choses qu'il veut saisir et les enl^ve." — BuiFon, 

 Histoire Naturelle, 4to, torn. xiii. p. 51. 



