72 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



Iii the Megatherium the inferior contour of the lower jaw is peculiarly re- 

 markable, as Cuvier has observed, for the convex prominence or enlargement 

 which is developed downwards from its middle part. In the Mylodon the corre- 

 sponding convexity exists in a very slight degree, not exceeding that which may 

 be observed at the corresponding part of the lower jaw of the Ai, or Orycterope. 

 A broad and shallow furrow extends along the outer side of the jaw, close to the 

 alveolar margin, from the beginning of the coronoid process to the anterior dental 

 foramen. 



The base of the coronoid process begins external and posterior to the last 

 grinder : the whole of the ascending ramus of the jaw, beneath the coronoid 

 process is excavated on its inner side by a wide and deep concavity, bounded below 

 by a well-marked ridge, which extends obliquely backwards from the posterior part 

 of the alveolus of the last grinder to the inferior margin of the ascending ramus, 

 which is bent inwards before it reaches the angle of the jaw. 



The large foramen or entry to the dental canal is situated in the internal con- 

 cavity of the ascending ramus of the jaw, two inches behind the last molar, three 

 inches from the lower margin of the ramus, and nearly five inches from the 

 elevated angle of the jaw: it measures nine lines in the vertical diameter, and its 

 magnitude indicates the large size of the vessels which are destined to supply the 

 materials for the constant renewal of the dental substance, — a substance which 

 from its texture must be supposed to have been subject to rapid abrasion. About 

 an inch behind the dental foramen a deep vascular groove, about two lines in 

 breadth, is continued downwards to the ridge which circumscribes the internal con- 

 cavity of this part of the jaw, and perforates the ridge, which thus arches over the 

 canal : this structure is present in both rami of the jaw. The mylo-hyoid ridge is 

 distinctly marked about an inch and a half below the alveolar margin. Other 

 muscular ridges and irregular eminences are present on the outer side of the base 

 of the ascending ramus, and near the angle of the jaw; as shown in fig. 1, 



PI. XIX. 



From the preceding descriptions it will be seen that the lower jaw of the 

 Mylodon is very different from that of the Megatherium ; with that of the Mega- 

 lonyx we have at present no means of comparing it. Among existing Edentata 

 the Mylodon, in the form of the posterior part and angle of the jaw, holds an in- 

 termediate place between the Ai and the great Armadillo ; in the form of the 

 anchylosed symphysis of the lower jaw it resembles most closely the Unau or 

 two-toed Sloth ; but in the peculiar external configuration of the symphysis 

 resulting from the mammilloid processes above described, the Mylodon presents 

 a character which has not hitherto been observed in any other species of Bruta, 

 either recent or fossil. 



In conclusion it may be stated, that the teeth and bones here described offer 



