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ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



present a more resisting structure than do those of the Mylodon ; having a larger 

 proportion of the dense ivory composed of the minute calcigerous tubes, and 

 a much smaller proportion of the softer external csementum ; in this respect the 

 Scelidothere recedes farther from Megathere, and approaches nearer the Arma- 

 dillos than does the Mylodon. 







The lower jaw resembles, in the general form of the posterior moiety which 

 is here preserved, that of the Sloth and Mylodon more than that of any other Eden- 

 tate species. Its deep posterior angle is produced backwards, and a broad coronoid 

 process rises and nearly fills the zygomatic space ; the condyle is flat, as the 

 glenoid surface has already indicated; its transverse diameter is an inch and 

 eight lines ; its antero-posterior diameter seven lines : it is principally extended 

 inwards beyond the vertical line of the ascending ramus. The lower contour of 

 the jaw describes an undulating line ; which, commencing from the posterior 

 angle, is at first gently convex, then slightly concave, then again convex, below 

 the alveoli of the teeth, where it is rounded and expanded, as in the Orycterope. 

 The fractured condition of the right ramus of this part fortunately exposed the 

 roots of the four grinding teeth, which constitute the dental series on each side 

 of the lower jaw. The length of the jaw occupied by these four alveoli is three 

 inches ten lines, which exceeds a little that of the opposed five grinders above; 

 the ramus of the jaw gradually diminishes in all its dimensions anterior to the 

 molar teeth ; the dental canal passes in a gentle curve below, and on the inner 

 side of the alveoli, whence it gradually inclines to the outer wall of the jaw. 



The whole ascending ramus of the jaw consists of a very thin plate of bone; 

 is slightly concave on the inner side, and the inferior margin of the produced 

 angle inclines inwards, as in the Mylodon and Sloth ; it is impressed on the outer 

 side with two shallow depressions, and two parallel ridges, both following the 

 gentle curvature of the part. There is a foramen on the outer side of the ramus 

 at the anterior part of the base of the coronoid process corresponding with that 

 in the lower jaw of the Mylodon, but the longitudinal channel which runs along 

 the outer side of the alveolar processes is wanting, and the expansion at the base 

 of those processes is more sudden and relatively greater ; the general correspon- 

 dence, however, between these lower jaws is such as would lead to the idea that 

 they belonged to animals of the same genus, were it not that the teeth present 

 modifications of form in the Scelidothere, as distinct from those of the Mylodon, as 

 are any of the minor dental differences on which genera or sub-genera of existing 

 Mammalia are founded in the present state of Zoological Classification. 



To make this distinction more readily intelligible, I have given a view of the 

 transverse section of the teeth in the right ramus of the lower jaw (fig. 4, PI. XXIII.), 

 corresponding with that of the Mylodon Darwinii, (PI. XVII., fig. 5). In the present 



it 



