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Show 1896.] MAMMALIAN DENTITION. 587 these have been separated from the lower jaws which Owen described under that name and placed in the genus Kurtodon (Athrodon) by Osborn (16), who first stated that they were not tuberculate, but now (16 a) apparently regards them as examples of trituberculate molars. In America, Marsh (11) has published the briefest note of the discovery of two upper jaws of Bryolestes and a single upper jaw of Biplocynodon (1 c, 8 cheek-teeth); these he has not figured, and his descriptions fail to show that they are tritubercular; in the case of Bryolestes he does not mention the cusps, while in Biplocynodon he mentions 5 cusps the arrangement of which does not suggest trituberculy. lu 1888 Osborn (16 & 16 a) described the upper molars of Kurtodon (see ante), Peralestes, Biplocynodon, and also of the Styla-codontia, under wdiich latter head he places Bryolestes, but on referring to this genus he states that the upper jaw is unknown ! In a later work (14) he only mentions the upper molars of Spalacotherium and of all the Amblotheriidae as being trituberculate ; evidently he refers Peralestes to Spalacotherium, as suggested by Lydekker (10), and Kurtodon to Amblotherium (Owen). These remarks will show what little material we have upon which to base the existence of the Jurassic tritubercular upper molar which is an essential feature in the tritubercular theory. A perusal of Osborn's (16) description of the upper molars of Peralestes shows, however, that they are anything but typical trituberculate teeth, for instead of possessing one internal and two external cusps arranged in a triangle, the inner cusp forming the apex, we find two interned, cusps1, of which the anterior is the largest, and a serrated ridge extending along the external border bearing several small cusps ; and as the anterior of these is slightly enlarged Osborn terms it the paracone, calling the two internal cones respectively the protocone (anterior) and the metacone (posterior). Now, according to the tritubercular theory, the metacone should be external and in a line with the paracone, not internal in a line with the protocone. Moreover, an examination of Osborn's figure and of the specimen show's that what he terms the paracone is here developed as an enlargement of the external cingulum and is not in any sense serially homologous with the metacone. A comparison of Osborn's two published figures of these teeth shows considerable differences in them, and on examining the actual specimen one finds that the figure in his large monograph (16) is the most accurate, the more frequently copied figure (13) being rather exaggerated in favour of trituberculism ; but with all he seems to have overlooked a small cusp on the antero-external shoulder of his protocone and between this main cone and this external paracone, which, to m y mind, far better suggests the anterior homologue of the metacone (see Pl. X X V I . fig. 33) and consequently the paracone from a tritubercular standpoint, although I believe this tooth to be capable of a totally different interpretation. If this tooth be compared with the molar teeth of the living Insectivora (figs. 34-36), it appears that the tuberculate external 1 The specimen shows three internal cusps, see fig. 33. oo |