70 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



Each tooth has a form and size peculiar to itself, and different from the rest, 

 but corresponds of course with its fellow on the opposite side. The same may 

 be observed, but in a less degree, in the teeth of the Megatherium itself; hence, 

 it is obviously hazardous to found a generic distinction upon a single tooth, 

 unless, as in the case of the Glyptodon* the modification of form happens to be 

 extremely well marked. The whole series of teeth, or their sockets, at least of one 

 of the jaws, should be known for the purpose of making a satisfactory comparison 

 with the previously established Edentate genera. 



The first molar in the present jaw is the smallest and simplest of the series: 

 its transverse section is ellipsoid, or subovate, narrowest in front, and somewhat 

 more convex on the outer than on the inner side : the long diameter of the ellipse 

 is nine lines, the short or transverse diameter six lines : the length of the tooth 

 may be about three inches, but I have not deemed it necessary to fracture the 

 alveolus in order to ascertain precisely this point. 



The second tooth presents in transverse section a more irregular and wider 

 oval figure than the first : the line of the outer side is convex, but that of the 

 inner side slightly concave, in consequence of the tooth being traversed longitudi- 

 nally by a broad and shallow channel or impression ; the longitudinal diameter of 

 the transverse section is one inch ; the transverse diameter at the widest part 

 nine lines. There is a slight difference in the size of this tooth on the two sides 

 of the jaw, the right one, from which the above dimensions are taken, being the 

 largest. 



The transverse section of the third tooth has a trapezoidal or rhomboidal 

 form; the angles are rounded off; the posterior one is most produced; the anterior 

 and posterior surfaces are flattened, the latter slightly concave in the middle ; the 

 external and internal sides are concave in the middle, especially the inner side, 

 where the concavity approaches to the form of an entering notch. The longest 

 diameter of the transverse section of this tooth is thirteen lines, the shortest seven 

 lines and a half: in the tooth on the right side the external surface is nearly 

 flat ; this slight difference is not indicated in the figure (PL XVIII.) 



The last molar, which is generally the most characteristic in the fossil Bruta, 

 presents in an exaggerated degree the peculiarities of the preceding tooth; the 

 longitudinal channels on both the outer and inner surfaces encroach so far upon 

 the substance of the tooth, that the central coarse ivory substance is as it were 

 squeezed out of the interspace, and the elevated ridge of the dense ivory describes 

 an hour-glass figure upon the triturating surface, the connecting isthmus being 

 but half the breadth of the rest of the tract; the external caementum preserves 

 nearly an equal thickness throughout. Of the two lobes into which this tooth is 



* See Proceedings of the Geological Society, March 1839, and Parish's Buenos Ayres, p. 178, £, PI. 1, fig. 2 

 and 3. 





