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Show 104 ZOOLOGY OF THE "\'OYAGE 017 THE BEAGLE. rally formed close to the fine uentine. In a few sit~ations I have obse~·ve~ o.ne of the medullary canals continued across the fwe dent111e, and anastomosmg With the correspond·w g canals of t 11 e c<emen t um. Tl1· e intcrspacc.s of the medu.l la. ry canals of the coarse dentine are principally occupied by calc1gerous tubes wh1ch have an !· l'rcgular course, anastomose 1·e t'I CH l"c•I 'l y, "••t J·u1 tei·Ininate. in very fin. e cells. The more rco·ular and parallel calcigerous tubes, which constttute the th111 layer of hard dent~1e are given off from the convexity of the terminal loops of the medullary canals. The course of these tubes (b. fig. l, Pl. XXXII.) is rat~cr more transversely to the axis of the tooth than the medullary canals from which they are continued. They run parallel to each other, but with minute undulations throughout their course, in which they arc separated by interspaces equal to one and a half their own diameter. As they approach the crementum they divide and snb-divide, and grow more wavy ami irregular: their terminal hranchcs take on a bent direction, and form anastomoses, dilate into small cells, nnd many are seen to beco111e continuous with the radiating fibres or tubes of the cells or corpuscles of the contiguous cremcntum. This substance enters largely into the constitution of the compound tooth of the Megatherium: it is characterized, like the cwmentum of the Elephant's grinder, by the presence of numerous radiated cells, or purkingian corpuscles, scattered throughout its substance, but may be uistinguislted by wide medullary canals which traverse it in a direction parallel with each other, and forming a slight angle with the transverse axis of the tooth. These canals are wider than those of the central coarse dentine, their diameter being riuo-th of an inch; they are separated by inter paces equal to from fom to six of their own diameters, divide a few times dichotomously in their course, and finally anasto'tnose in loops, the convexity of which is directed towards, and in most cases is in close contiguity with, the layer of dense dentine. Fine calcigerous tubes are every where given off at right angles from the mequllary canals of the crementum, which form a rich reticulntion in their interspaces, and a direct continuation between the loops of the medullary canals and tile calcigerous tubes of the dense dentine. The crementum differs from the coarse dentine in the larger size and wider interspaces of its medullary canals, and by the presence of the bone-corpuscles in their interspaces; but they are brought into organic communication with each other, not only by mean of the tubes of the dense dentine, but by occasional continuity of the medullary canals across that substance. The tooth of the M egatherium thus offers an unequivocal example of a course of nutriment from the dentine to the crementulll, ami reciprocally. Retzius observes with respect to the human tooth, that "the fine tubes of the cremcntum enter into immediate communications with the cells aud tnbes of the dentine (zahnknochen), so that this part can obtain ft·om without the requi- FOS IL l\IAl\11\IALIA. J05 s~te humours after the central pulp has almost ceaseu to exist." In the lVIcgathenum, however, those anastomoses have not to perform a vicarious office since th . pu~p ~l,aint~ins its full size and functional activity during the whole period of tit~ anu~a ~existence. ~t rela.tes to the higher organized condition, and greater degree of VItahty of the enttre gnnder in that extinct species. The co~ical cavitie~ (d. Pl. XXXI.) attest the size and form of the persisteut pulp; .the dtameter of 1ts base is equal to the part of the crown of the tootlt wluch 1s fo~·med by the co~rse and ilne dentine. 1:;-rom the gmdual thinning oft: and final disappearance of these substances as they reach the base of the tooth, l conclude that they were both formed at the expense of the pulp. The fine tubes and ce~ls must have been excavated in its peripheral layer for the reception of the hardcmng salts of th~ dense. dentine, .and the rest converted into the parallel serie. of medullary canals With thcll' respective systems of calcigcrous tubes, in a mannet· ~losely analogou~ to ~he developm.ent of the entire tooth of the Oryctcropus. fh~ coarser dcntwe of the tooth of the Megatberium dift'ers, in fact, from the ent~re.tooth of. the Orycteropus, only in that the parallel medullary cauals and their r~ut.atwg calc1gerous tubes arc not separated from the contiguous canals by a OJ lmct layer of cremcntum, anti that the medullary canals anastomose at their peripheral extremities. The wide spaces, (e. Pl. XXXI.) indicate tile thickness of the dental capsule by the ossification of which the exterior stratum of cement was f~rmed. Jt was not until I knew the true structure of the tooth of the Megatltcnum, that I could comprehcud the mode of its formation. The parallel layers of enamel in the Elephant's grinder arc formed, as is well known, by mcmbmnou~ plates passing from the coronal end of the closed capsule towards the base of the tooth ; hut a certain extent of enamel can o11ly thus be formed, and when the crown of the grinder has once protruded, and come into use, the enamel cannot be addeu to. rfhe modification of the structure of the tooth of the Megatherinm readily permits the unintcnuptcd and continuous formation of the dense substance wl1ich is analogous to the enamel of the Elephant's grinder. With respect to the question of the respective allluitie:s of the Megatherium to the Bradypodoid or Dasypodoi<.l families, the result of tllis examination of the teeth speaks strongly foi' its closer relationship with the former group: the Megalonyx, Mylodon, anti Scelidotherium, in like manner concspond in the stmcture of their teeth with the Sloth, and differ from the Armatlillo. If from a similarity of dental structure we may predicate a similarity of food, it may rcasonahly be conjectured that the leaves and sof't succulent sprouts of trees may have been the staple diet of the Mcgatherioid q uadmpeds, as of the · existing Sloths. Their enormous claws, I concluue, from tltc fossorial character of the powerful mechanism by which they were worked, to have been employed, p |