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Show 82 ZOOLOGY OF TilE VOYAGE OF TilE BEAGLE. . . d those of the Mylodon i having a larger present a more res1stmg structure than ° f 1 · te calcig·erous tubes and d · composed o t 1e mmu ' proportion of the ense Ivory . 1 entum . in this respect the a much smaller proportion of the softer cxterna crem . l , nearer the ArmaScelidothere recedes farther from Megathere, and appiOac leS (lillos than does the Mylouon. . osterior moiet which The lower jaw resembles, in the general form of the P y Ed _ l ·s here preserved that of the Sloth and Mylodon more than that of any otdher e~d tate species. Its d' eep posterior angle · db 1 ·d d a broa corono1 IS produce ac (Wai s, an . fl 1 l t' ace· the condyle 1s at, as t 1e process rises and nearly fills t 1e 1.ygoma LC sp ' . . . glenoid surface has already indicated; its transverse d1~met~r ~s an mch and eight lines· its antero-posterior diameter seven lines: it IS prmctpally extended inwards be~ond the vertical line of the ascending rum us.. The lower contoUI:. of the jaw describes an undulating line ; which, commencmg fro~ the postetiOr angle, is at first gently convex, then slightly concave, then a~am couv~x, b~low the alveoli of the teeth, where it is rounded and expanded, as m the OtycteJOpe. The fractured condition of the right ramus of this part fortunat:ly exposed ~he roots of the four grinding teeth, which constitute the dental senes on ~a.ch s1de of the lower jaw. The length of the jaw occupied by these four a~veoh JS three inches ten lines, which exceeds a little that of the opposed five grmdeJ.'S above i the ramus of the jaw gradually diminishes in all its uimensions antenor t? the molar teeth · the dental canal passes in a gentle cnrve below, and on the mner side of the ~lveoli, 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; it 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 correspondence, 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 ofform 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, Pl. XXIII.), corresponding with that of the Mylodon Da1·winii, (Pl. XVII., fig. 5). In the present FOSSIL II[AMMALIA . 83 sub-genus tl~e antero-pos~erior e~tent of the four alveoli of the lower jaw nearly equals four mches, and IS relatively greater than in the Mylodon, although the teeth are placed cJoser together; this is owing to their greater relative size. The ~rst ~olar tooth presents the simplest form ; its transverse section is a compressed meq.uJlater~l tr~angle with the angles rounded off; the longest diameter of this sectwn winch Is parallel with the inner alveolar border is eleven lines, the trans~ erse diameter almost six lines ; the base or broadest side of the triangle is turned mwards, and is slightly concave; the two smaller sides are also slightly concave. The second molar is placed more obliquely in the jaw; the long axis of its transvet·se section intersects at an acute angle that of the jaw itself; the transverse section presents a compressed or oblong form, with the larger end next the outer side, and the smaller end next the inner side of the jaw ; this end is simply rou~ded, but the outer end presents a sinuosity, corresponding to a broad groove whtch traverses the whole length of the outer side of the tooth · the anterior which corresponds to the internal side or base of the transverse ~ection of th; preceding molar, is slightly concave. . The third molar has nearly the same form and relative position as the precedmg; the long diameter of the transverse section is, in both, ten lines and a half; the principal transverse diameter is, in the second molar five lines, in the third nearly six; the difference of form observable in these as compared with the two middle grinders of the Mylodon is well marked ; in the latter these teeth are impressed with a longitudinal groove on their inner sides; in the Scelidothere they have a similar impression along their outer but not along the inner side. In the last molar the resemblance is much closer, and the modification of form by which it differs from the preceding ones is of the same kind ; the transverse section gives an irregular oblong figure with its axis nearly parallel with that of the jaw, and constricted at the middle by sinuo ities produced by two wide channels which traverse longitudinally, one the outer, the other the inner side of the tooth ; the Iutter groove is much wider and shallower in the Sceliclothere than in the Mylodon. The two lobes produced by these grooves are more equal in Scelidothere; the anterior one is concave on its anterior surface instead of convex as in the Mylodon ; the posterior one is more compressed; the longitudinal or antero-posterior diameter of the transverse section of this tooth is one inch five lines ; the greatest transverse diameter is nine lines; the diameter of the isthmus joining the lobes is three lines and a half; the entire length of this tooth is three inches three lines.* • It requires little stretch of imagination to conceive that this moro complex posterior tooth (Pl. XXIII, fig. 4, 4) in tho lower jaw is the representative of tho two smaller posterior teeth (ib. fig. 3, 4, and 5) of the upper jaw conjoined. M2 |