OCR Text |
Show 102 ZOOLOGY OF TilE VOYAGE OF TilE DEAGLE. 'd · ·p1 XXX fig 2) The vertical diame-to air derived from the spheno1 smuses, ( · · · · . . . f , h · 1 a ·1ty 1 • ter o t c cmnta c v 8 1/!' out· 1'nches • eion· ht lines; its transverse. d~am.e tcr, wluch is greatest in the posterior third part of the cavity, c~rr~spo~dmg ~vlt.h the pos-terior part of the cerebrum is six inches: from the mdJCatwns affolded by tl~e · f' th · 1 av1'ty 1· remams o e cram a c 11 Mr· . Darwin's specimens, I conclude that the bram of tho Meo-atherium was more depressed, and upon the whole, small_er by nearly one-half than that of the Elephant; but with the cerebellum relatively larger, and situated more posteriorly with relation to the cerebral hemisphe:es: :Vhence it may be concluded that the Megatherium was a creatme_ of ~ess_ mteliigence, and with the command of fewer resources, or a less vaned mstmct than the Elephant. It has been usual to characterize the Megatherium, in conformity with the concmrcnt descriptions of Bru, Cuvier, and D'Alton, by the dental formula of nwlares {- ~. i.e. by the presence of four grinding teeth on each side of the upper, as of the lower jaw. It was the agreement of the excellent authorities above cited in this statement, which induced Mr. Clift and myself to regard a single detached tooth, which formed part of the valuable collection of remains of the Megatherium deposited in the Hunterian Museum by Sit· Woodbine Parish, as being, ft-om its co111paratively small size, the tooth of either a younger individual or of a smaller species of Megatherium. Upon clearing away the matrix from the palatal and alveolar sm-face of one of the cranial fragments of the Megatherium in Mr. Darwin's collection, I was gratified by the detection of the crown of a fifth molar, corresponding in size and form with the detached tooth, above alluded to: its small size, and its position have doubtless occasioned its being over-looked in the cranium of the great skeleton at Madrid. Tho anterior molar of the upper jaw presents a nearly semicircular transverse section, with the angles rounded off; the three succeeding teeth are foursided, with the transverse somewhat exceeding the antero-posterior diameter : they are rather longer ami larger than the first: the last molar is likewise foursided, but pres;ents a sudden diminution of diameter, and is relatively broader. The following are the respective dimensions of the upper maxillary teeth. First Molar. Second Molar. Third Molar. Fourth Molar. Fifth Molar. In. Lince. ln. Lines. In. Lines. In. Lines. In. Lines. Length G 4 4 7 5 Transverse diameter . 1 4 3 0 1 4 Antero-posterior diameter . 1 5 0 0 1 11 0 10 Besides the differences in size, the upper molars vary as to their curvature : this difference is _exhibited in the vertical section of these teeth figured in Pl. XXXI. The convexity of the curve of the first, second and third molars is directed FOSSIL l\'lAMMALIA. 103 forw. ard.s ; the fou. rth is . strai. ght, its anterior surface only describi11g a sl't g ht con-v_ extty Ill the vertical dtrectwn ; the fifth tooth is curved, but in a contrary direc-tw~ to the others; an~ the _bases of the five molars thus present a general convetgence towards a pomt a little way behind the middle of the series. . The next ~eculiarity to be noticed in these remarkable teeth is the great length ~f tl~e pu.lp-cavtty (d), th~ apex _of which is p_arallel with the alveolar margin of the JaW· _a tlansverse fissure 1s contmued from thts apex to the middle concavity of the workmg surface of the tooth, which is thus divided into two parts. Each of these parts consists of three distinct substances,-a central part analogous to the body or bone of the tooth or 'dentine,' a peripheral and nearly equally thick layer of ccementum, and an intermediate thinner stratum of a denser substance wh~ch is described in Mr. Clift's memoir on the Megatherium as 'enamel,' and t~ ~htch s~1bstance in the compound teeth of the Elephant, it is analogous both in Its relative situation, and relative density to the other constituents. Microscopic examinations of thin and transparent slices of the tooth of the Megathcrium prove, however, that the dense layer separating the internal substance from the crementum is not enamel, but presents the same structure as the h~rd 'dentine' or ivory of the generality of Mammalian teeth; and corresponds w~th the thin cylinder of hard 'dentine' in the tooth of the Sloth. No species of the Order B1·uta has true enamel entering into the composition of its teeth . hut t~e modifications of structure which the teeth present in the different gener~ of Hus order are considerable, and their complexity is not less than that of the enamelled teeth of the Herbivorous Pachyderms and Ruminantia, in consequence of the intro_duction of a dental substance into their composition corresponding in structure wtth that of the teeth of the Mgliobates, Psammodus, and other cartilaginous fishes. . The microscopic investigation of the structure of the teeth of the Megathe-rmm was undertaken chiefly with the view of comparing this structure with th~t of the teeth of the Sloth and Armadillo, and of thus obtaining an insight into the food, and an additional test of the real nature of the disputed affinities of the Megatherium. The central part of the tooth (c. Pl. XXXI.) consists of a coarse ivory, like the corresponding part of the tooth of the Sloth. It is traversed throughout by medullary canals nhth of an inch in diameter, which are continued from the pulp-cavity, and proceed, at an angle of 50°, to the plane of th dense ivory, parallel to each other, with a slightly undulating course, having regular interspaces, equal to one and a half diameters of their own arere, and generally anastomosing in pairs by a loop of which the convexity is turned towards the origin of the tubes of the fine dentine, as if each pair so joined consisted of a continuous reflected canal, (c. fig. 1, Pl. XXXII.) The loops are gene- |