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Show 18 ZOOLOGY OF TilE VOYAGE OF THE BEAGLE. . . fi s 2 and 3, Pl. IV.) was found by itself, em- This latter tooth (Fig. 3, Pl. 1 ·: Tg · . Cat·carana near the Parana, at the . h b k f the Rw ercero, 01 ' . bedded m t e an so . ht miles from the locality where the head was dis-distance of a hundred and eig y th f Toxodon apparently the seventh of the . d F . ments of a molar too o a ' . . cove1e . 1ag .c d B . d de Sta Fe in the provmce of Entre left side upper jaw, were also !OUn at aJa a . ' Rios, di~tant forty miles from the mouth of the Rw Tei:cero. * as in most of All the molar teeth are long and curved, and without fangs, . . t h e h er bI.V OrOUS S peci'es of the Rodent Order : in those, . however, with't cmf vtle d rinders as the Aperea or G um. ea-pi.g , and Cavia Pataclwnzca, the conc·a vi 'yd o dl'e g 'inders is directed outward, the fangs of the teeth of the opposite. st es Iupper gr . . b t in the Toxodon, the convexity of the verging as they ascend m the sockets ' u ' h . ddl 1' rinders is outward, and the fangs converge and almost mee.t at. t e mi .e .me ~f the palate, forming a series of arches, capable of overcommg 1mmen';:'i::s~~~ ance from pressure. (See the upper view of the skull, Plate III., m w fractures expose to view a part of the series of these arched s?ckets.) . . Of the incisors the two small ones (the sockets of which ~re mdiCated at s s Pl. III.) are sit~ated in the middle of the front of the upper J.aw, cl~se to the sut' ure between the m· termaxt·l lan·e s, an d t 11 e two 1a rge on es in Immediate con-tiguity with the small incisors, which they greatly exceed in. size. The sock~ts of the two large incisors (t t, Pl. III.) extend backwards, m an arche~ fOI:, preserving a uniform diameter, as far as the commencement of the alveoli oft .e molar teeth: the curve which they describe is the segment of a circle; the position, form, and extent of the sockets of these incisors are the same as in those of the corresponding teeth of the Rodentia. . The matrix, or secreting pulp of the large incisors, was lodged, as m the Rodentia, in close proximity with the sockets of the anterior molars; and we are enabled to infer, from the form of the incisive sockets, notwithstanding the absence of the teeth themselves, that the pulp was persistent, and that the growth of these incisors like those of the Rodentia, continued throughout life. This condition, joined with the form and curvature of the socket, implie~ a continual wearing away of the crown of the tooth by attrition against op~osmg incisors of a corresponding structure in the lower jaw: and as a corollary, It may be inferred that the teeth in question had a partial coating of enamel, to produce. a cutting edge, and were, in fact, true dentes scalpra1'ii. The number of incisors m the upper jaw of Toxodon, is not without its parallel in the Rodent Order, ~he genus Lepus being characterized by four, instead of two superior incisors, whtch also present a similar relative size but have a different relative position, the * True fangs exist only in teeth of temporary growth, they may be one or more in number, but alway8 diminish in size as they recede from the crown of the tooth, and aro either solid, or with a. very small canal. FOSSIL MAMMALIA. 19 small incisors, in the hare and rabbit, being so placed immediately behind the large pair, as to receive the appulse of the single pair of incisors in the lower jaw. In the Toxodon the position of the incisors, in the same transverse line, might lead to the inference, that they were opposed by a correspoi?-ding number in the lower jaw; but the numerous examples of inequality, in the number of incisors, in the upper and lower jaws of existing mammalia, forbid any conclusion on this point.* The sockets of the small mesial incisors of the Toxodon (s s, Pl. III.) gradually diminish in size, as they penetrate the intermaxillary bones, and we may, therefore, infer that the pulp was gradua11y absorbed in the progress of their development; and that, like ordinary incisors, their growth was of limited duration, and their lodgment in the jaw effected by a single conical fang. I may observe, that the formation of a fang is the necessary consequence of the gradual absorption of the matrix or pulp of a tooth ; for the pulp continues, as it diminishes in size, to deposit ivory upon the inner surface of the cavity of the tooth from which it is receding, and the tooth or fang thus likewise progressively diminishes in size. The formation of the socket proceeds uninterruptedly, and the bone encroaching upon the space left by the tooth, closely surrounds the wasting fang, and affords it a firm support; and thus an inference may be drawn from the form of the socket alone, as to whether the tooth it contained had or had not one or more conical fangs, and consequently whether its growth was temporary or uninterrupted. Applying this reasoning to the molar teeth of the Toxodon, we infer that their growth, like those of most of the Phytiphagous Rodents, of the Megatherium and Armadillo, was perpetual, because their sockets are continued of uniform size from the open to the closed extremity; and the molar tooth which is preserved proves the accuracy of the deduction, inasmuch as its base is excavated by a large conical cavity for the lodgment of the pulp, the continued activity of which was the compensation here designed to meet the effects of attrition on the opposite or grinding surface of the tooth. The molar tooth discovered by Mr. Darwin in the banks of the Tercero, not only belonged to the same species as the skull under consideration, but to an individual of the same size; it fits exactly into the socket next to the posterior one of the right side. The figures subjoined of this molar tooth (Fig. 3, Pl. I.; figs. 2 and 3, PI. IV.) almost preclude the necessity of a description. The transverse section of the tooth gives an irregular, unequal sided, prism; the two broadest sides of which converge to the anterior angle, which is obtusely rounded. The * This was written before an examination of the fragment of a lower jaw, forming part of Mr. Darwin's collection of Fossil Remains, hacllcd me to suspect that it was refcrriblo to tho genus Toxodon; should this suspicion prove correct, the four unequal incisors of the upper jaw are opposed to six equal sized ones in the lower. |