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Show 586 ON A MAMMALIAN TOOTH FROM THE WEALDEN. [NOV. 17, far as possible by the skilled hand of Mr. Richard Hall, but one still remains attached and is thus obscured. In side view (fig. 1, A ) tbe low enamelled crown is shown supported by two roots, which are robust, of nearly equal size and depth, incompletely separated in the upper half and terminating obtusely. So far as can be ascertained these are tbe sole roots, being apparently almost as wide as the crown. The coronal surface (fig. 1, B) is somewhat longer than broad, half as wide at one end as at the other, with one of the lateral margins rounded, almost angulated. It is much abraded, evidently the result of wear during the life of the animal, and the enamel is thus removed except quite at the border. The highest elevation (x) is large and obtuse, occupying one half of the gently rounded lateral margin of the tooth, and connected with a short spur which extends to the middle of tbe crown. There is evidence of a smaller elevation, crescentic in form, on the margin of the other half of the tooth ; and the lateral margin opposite and adjacent to the most elevated portion forms a slight rim with feeble traces of crenulated enamel. A detached tooth, and more especially a worn tooth, certainly does not suffice for generic and specific determination. The most striking feature of the specimen now described, however, is its close resemblance to the multituberculate Microlestes-type of molar, so common among Mesozoic Mammals and apparently retained in the true dentition of the existing Monotreme, Ornithorhynchus \ It only seems to differ in the extraordinary amount of wear to which the crown has been subjected, and in the appearance of this abrasion not having been produced entirely by an upward and downward or antero-posterior motion, of which the jaws of the known Multituberculata seem to have been alone capable. This apparent divergence of character may, nevertheless, be deceptive ; and on comparing the Wealden tooth with the posterior true molar of tbe Purbeckian genus Plagiaulax (fig. 2, p. 585), the extent of the correspondence in the configuration of the coronal aspect is certainly remarkable. One lateral border (the inner) is much more elevated than the other (the outer) in both cases; and the largest cusp in each is at the antero-inner angle. In the absence of further evidence, it thus appears advisable to assign the new tooth to the genus Plagiaulax, of which it will represent a species larger than any hitherto described. If, moreover, the specimen should prove to be the ultimate lower molar of this genus, it is sufficiently distinguished from any known form by the shape of the crown ; and, until tbe acquisition of further material, the species in question may bear the provisional name of Plagiaulax dawsoni, in honour of its discoverer. Mr. Dawson has already enriched the British Museum with many fine portions of the Wealden Reptilia; and there is now much hope that his further researches in the bone-beds will add the long-expected series of Wealden Mammalia. 1 O. Thomas, Proc. Eoy. Soc. 1889, vol.xlvi. p. 126. |
