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Show J 70 ZOOLOGY OF THE VOYAGE OF 'l'IIE DEAGLE. 1. 't If and different from the rest, Each tooth has a form ancl size pecu mr to 1 se ' '. . Th • .e ll tl PI)OSite side e same may but col'l'esponds of course with Its Ie ow on 1e o .' . If h . 1 h f the Megathenum 1tse ; ence, be observed but in a less degree, m t le teet 0 . . th it is obvio~sly hazardous to found a generic distinctiOn upon a smgle too b' . 1 d'fi . t' of form happens to e unless, as in the case of the Glyptorlon,* t 1e mo 11ca 10~ 1 f extremely well marked. The whole series of teeth, or theu so~kets, at easto .one of the jaws, should be known for the purpose of making a satisfactory companson with the previously establishe<l Edentate genera. . The first molar in the present jaw is the smallest an~ si~plest of the senes: its transverse section is ellipsoid, or subovate, narrowest m front, and some':hat more convex on the outer than on the inner side: the long diameter of the elhpse 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 wtder oval figure than the first : the line of the outer side is ~onvex, but that ~f th.e inner side slightly concave, in consequence of the tooth bemg .trav~rsed :ongitud•nally by a broad and shallow channel or impression; the longttudmal d~ameter of the transverse section is one inch; the transverse diameter at the w1dest 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 <limensions are taken, being the largest. The transverse section of the third tooth has a trapezoidal or rhomhoi<lal torm; the angles are rounded off; the posterior one is most produce<l; 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 inuer side, where the concavity approaches to the form of an entering notch. The longest uiameter 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 fiat; this slight difference is not indicated in the figure (Pl. XVIII.) The last molar, which is generally the most characteristic in the fossil Bn.tla, 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 hut half the breadth of the rest of the tract; the extemal c::ementnm preserves nearly an equal thickness throughout. Of the two lobes into which this tooth is * Sec Proceedings of the Geological Society, March 1839, and Parish's Buenos Ayres, p. 178, b, Pl. 1, fig. 2 and 3. FOSSIL MAMMALIA. 71 divided by the transverse constriction, the anterior is the largest; their proportions and oblique position are pretty accurately given in the figure. The longitudinal diameter of the transverse section of this tooth is one inch, seven lines, its greatest lateral or transverse diameter is ten lines, its least diameter at the constricted part is three lines, the length of the entire tooth is four inches. Judging from the form of the jaw, the length of the other teeth decreases in a regular ratio to the anterior one. The posterior tooth is slightly curved, as shown in fig. 2, Pl. XIX., with the concavity directed towards the outer side of the jaw. The general form of the horizontal ramus of the jaw, is so well illustrated in the figures Pl. XVIII. and XIX., that the description may be brief. The symphysis is completely anchylosed, about four inches in length, and extended forward to the extremity of the jaw at a very slight angle with the inferior border of the ramus: it is of great breadth, smooth and gently concave internally, and suggests the idea of its adaptation for the support and gliding movements forwards and backwards of the free extremity of a long and well-developed tongue. The exterior snrface of the symphysis is characterized by the presence of two oval mammilloid processes, situated on each side of the middle-line, and about half way between the anterior and posterior extremes of the symphysis. A front view of these processes, of the natural size, is given in fig. 4, Pl. XIX.: a side view of the one on the right side represented in the rednced figure. Nearly four inches behind the anterior extremity of the above process is the large anteriot· opening of the dental canal: it is five lines in diameter, situated about one-third of the depth of the ramus of the jaw from the uppe•· margin. The magnitude of this foramen, which gives passage to the nerve and artery of the lowet· lip, indicates that this part was of large size; and the two symphyseal processes, which probably were subservient to the attachment of large retractor muscles, denote the free and extensive motions of such a lip, as we have presumed to have existed from the size of the foramina destined for the transmission of its nervous and nutrient organ . The angle of the jaw is pt'O!luce<l backwards, and ends in an obtuse point, slightly bent upwards; a foramen, one-third less than the anterior one, leads from near the commencement of the dental canal, to the outer surface of the jaw, a little below and behind the last molar tooth; this foramen presents the same size and relative position on both sides of the jaw. I find no indication of a conesponding foramen, or of symphyseal processes in the figures or descriptions of the lower jaw of the Megatherium, nor in the lower jaw of the Sloths, Ant-eaters, Armadillos, or Manises, which I have bad the opportunity of examining with a view to this comparison. |
