5S6 ON A MAMMALIAN TOOTH FROM THE WEALDEN. [Nov. 17, 



far as possible by the skilled hand of Mr. Richard Hall, but one side 

 «till remains attached and is thus obscured. In side view (fig. 1, A) 

 the 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 the 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 (a;) 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 the crown. There is evidence of a smaller eleva- 

 tion, crescentic in form, on the margin of the other lialf 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-tjpe of molar, so common among 

 Mesozoic Mammals and apparently retained in the true dentition of 

 the existing Monotreme, Ornithorhyncfnis \ 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 the Purbeckian genus Plaginulax (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-iuner angle. 



In the absence of further evidence, it thus appears advisable to 

 assign the new tooth to the genus Plagiaulax, of which it will repre- 

 sent 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 the 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. Tbomas, Proc. Eoy. Soc. 1889, vol.xlvi. p. 126. 



