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Show 588 MR. M. F. WOODWARD ON jMav 5 cingulum seen in Peralestes is comparable with the similar structure so frequently present in this group, and well exemplified in the upper molars of Talpa. If so, it becomes further evident that the two larger cusps of Peralestes represent the paracone and metacone of these living forms, these cusps being commonly developed quite a long distance from the external border of the tooth (Talpa, fig. 35). Consequently the internal shelf, which we have seen in living Insectivores bearing the proto- and hypocone, is not developed in upper molars of Peralestes. If this comparison is correct, we are justified in concluding that the upper molars of this fossil form were not tritubercular in the sense understood by the supporters of the Cope-Osborn theory, and, further, those of Kurtodon being undoubtedly ridged and not tuberculate, while those of Bryolestes and Biplocynodon are either undescribed or possess 5 cusps, we consequently have no palseontological evidence to support the assumption that a tritubercular stage was passed through by the mammalian upper molar in its evolutions from a protodont or possibly a triconodont tooth. Under these circumstances I see no reason to believe that the primitive cone must necessarily occupy an antero-internal position such as Osborn's protocone does. Palaeontological evidence being then wanting or so fragmentary, we are obliged to fall back on the less torn pages of ontogeny. On doing so, we find that the upper molar cusp, which develops first and as a direct continuation of the dental germ in the majority of the Mammalia, is the antero-external or paracone: this I think is strongly in favour of the view put forward by Eose (19), that the paracone is the most primitive cusp, though I think it would be rather confusing to apply Osborn's term " protocone " to it, seeing that this term has been already applied to another cusp in the same tooth. Of the primitive nature of the paracone we have slight palaeontological evidence if, as I have suggested, the largest cone of the Peralestes upper molar (Osborn's protocone) is the homologue of the paracone of living Insectivores. But if we further include the molariform premolars in our study, we find this view is supported both by ontogenists (22) and palaeontologists, for Scott (21a) has proved, and Osborn and Wortman (32) have accepted his conclusions, that the antero-external cone in these teeth is the primitive one from a palaeontological standpoint, and Taeker has shown in the Ungulates, and I myself in tbe Insectivora, that tins antero-external cone in the premolars develops first in the ontogeny of the premolar cusps. With regard to the tritubercular upper molars of the Centetim Ac. (fig. 34, a & b), I should conclude that the main cone of this type of tooth, usually termed the protocone, was really the paracone: the whole tooth representing only the antero-external triangle ot such a form as Talpa (fig. 35, a & b), i. e. the crescentic paracone with its two external cingulum cusps, the two last named being commonly but incorrectly described as the para- and meta-cone in Centetes: that in the Centetidce no marked indications of the protocone |