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Show Z OOLOGY OF TilE VOYAGE OF THE BEAGLE. ()4 . . and used it in a generic sense, Cuvier offet·~d no retamed the name of JJfegalonyx, . . ht b generically either distingmsheu characters whereby other fossil rematns mJJg e .. less among such remains from or .t dent1'f1i e d 'tl th JJfen·alonyx qffersonn, un ' Wl 1 e o d' 'th the descriptions ' h . . law exactly correspon mg Wl there happened to be a toot ' 01 ~ c . a 8 ecific identit , and and figures in the Ossemens Fosstles; and when, o.f comse, p y not m rel a generic relationship would be established. . The gyreater part of Cuvier's chapter on lJ'Iegalonyx is devoted t? ~he beaudtlful . 1 h 1 whereby It 1s prove to and justly celebrated reasonmg on the ungu~a p. a anx, . d but belon()' not to a gigantic Carnivore of the Lwn-kmd, as .Jefferson. suppose ' to the0 less formidable order of Edentate quadrupeds i and Cuvier, m reference to the tooth -the part on which alone a generic character could have been founded, -mere\; observes that it resembles at least as much the teeth of one of the great Armadillos, as it does those of the Sloths. iff • : In the last edition of the Regne Animal, Cuvier mtroduces the Megat!terwrn and Megalonyx, between the Sloths and Armadillos; but alludes to no other difference between the two genera than that of size,-" l'autre, le Megalonyx, est un peu mom· d r e ." (p . 226 .) Some systematic naturalists, as Desmarest, a.n d Fischet·, have, therefore, suppressed the genus, and made the Megalonyx a spec1es of Meuatfte1·ium under the name of Megatlteriurn Jqffersonii. The dental characters of the genus Meo·atlterium are laid down by Fischert as follows:-" Dent. pTim. et Zan. l molare~ -t-t, obducti, t1·itores, coronide nunc plana transversim sulcala nunc medio excavata marginibus prominulis." That Megalonyx had the same number of molares as JJfegatlterium, (supposing that number in the Megathere to Le correctly stated, which it is not,) is here assumed from analogy, for neither Jefferson, Wistar, nor Cuvier,- the authorities for Megalony:c quoted by Fischerpossessed other means of knowing the dentition of that animal than were afforded by the fragment of a single tooth. Now the almost entire lower jaw about to be described offers, in so far as respects the general form and structure of the teeth, the same kind and degree of correspondence with the Megatlterium, as does the J.lfegalonyx J riffersonii of Cuvier: and, what is only probable in that species, is here certain, viz., an agreement with the Megatherium in the class, viz. molares, to which the teeth exclusively belong. The question, therefore, on which I find myself, in the outset, called upon to come to a decision is, as to the preference of the mode of viewing the subject of the generic relationship of the Megalonyx adopted by Desmarest, Fischer, &c., or of that, on which Cuvier, and after him Dr. Harlan, have practically acted : whether, in short, the genus JJfegcctlterium is to rest upon the more • Speaking of this tooth, Ouvicr ol>scrvcs, "Je l':wois cru d'abord neccssairemcnt de paresseux; mais aujourdhui que je connois mieux l'osteologie des divers tatous, jc trouve qu'elle ressemble nu moins an taut a une dent de l'un des grands tatous.-Loc. cit. p. 172. t Synopsis Mammalium. FOSSIL MAMMALIA. (j5 comprehe~sive characters of kind and general structure of the teeth, or upon tbe mor~ rcstncted ones, of form and such mouifications in the uisposition and proportiOns of the component textures of the tooth, as give rise to the characteristic appearances of the triturating surface of the crown. With respe~t to existing Mammalia, most naturalists of the present day seem ~o b.e :mar.nmous as to the convenience at least of founding a generic or subgenenc d1stmct10n on well marked modifications in the form and structure of the teeth, although they may correspond in number and kind, in proof of which it needs only to peruse the pages of a Syslema JJiarmnalium which relate to the uis~ribution of. the Rodent Order. According to this mode of viewing the l~gtcal abstractiOns .under which species are grouped together, the extinct Edentate Mammal discovered by Jefferson must be referred to a genus distinct from Megat!teriurn, and for which the term Meo·alonyx should be retained. This will be sufficiently evident by comparing th: descriptions given by Cuvier of one of the teeth of the lJfep,·alonyx Jqj'ersonii, and by Dr. Harlan of a tooth of his Megalonyx laqneatus, with those of the lJfegat!terium which have been published by Mr. Clift. The fragment of the molar tooth of the lJfeo·alontJX Jen'ersonii de- . 5 J ':11' ' ~cnb~d and figured in the Ossemens Fossiles, seems to have been implanted in the Jaw, 1tke the teeth of the Megatlteri'um, by a simple hollow base similar in form and size to th~ protruded crown : its structure Cuvicr describes as consisting of a central cylmdcr of bone enveloped in a sheath of enamel.'~ The transverse section of this tooth presents an irregular elliptical form, the external contour bein<Y gently anu uniformly convex, the internal one, unuulating; convex in the middle~ and slightly concave on each side, arising from the tooth being traversed lon()'itu-dinally on its inner side by two wide and shallow depressions. 0 The imperfect tooth of the species called by Dr. Harlan Megalonyx laqueatus, and of which a cast was presented by that able and industrious naturalist to the Museum of the Royal College of Surgeons, resembles in general form, and especially in the characteristic double longitndinal groove on the inner siue, the tooth of the Megalvnyx Jqjj'ersonii. It is thus described by Dr. Harlan: "The fractured molar tooth appears to have belonged to the inferior maxilla on the right side; the crown is destroyed; a part of the cavity of the root remains. The body is compressed transversely, and presents a double curvature, which renders its anterior and exterior aspects slightly convex; the posterior and interior gently concave; these surfaces are all uniform, with the exception of the interior ot· mesial aspect, which presents a longitudinal rib or ridge, one-half the thickness of the long diameter of the tooth; with a broad, not profound longitudinal * It is most probable that the substance which is here termed "ennmcl," is similar to that which forms the dense promiucut ridges in the tooth of the 1\fegatherium, and which I have shown to be composed of minute parallel caleigerous tubes, similar to the ivory or bone of the human tootlt. K |