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Show 1893.J \yR. C. J. FORSYTH MAJOR ON MIOCENE SQUIRRELS. - 205 ceeds from the inner side outwards in superior, and from the outer side inwards in inferior molars. 5. The now prevalent transverse arrangement of cusps or lobes is not the primitive condition, but a specialized pattern of the crown. The first two points need no discussion, as no biologist of the present day denies them. I thirdly maintain that the more brachydont a molar is, the more polybunous it is ; so that change, which in our case means reduction, simplification, of the molar-crown elements, goes hand in hand with the gradual progress from brachy-donty towards hypsodonty. In the general survey of Sciurine teeth, it has already been shown that the more the molars are brachydont, the more they are polybunous, so that by this statement alone polybuny is proved to be the primitive condition. If we examine the outer parts of upper and the inner parts of lower molars, w e see that they present much less variation in Brachydontia as compared to Hypsodontia, and in the various stages of Hypsodontia compared together, than does the rest of the crown, especially the inner side in upper and the outer side in lower molars. It therefore at once strikes us, that the outer side of upper and the inner side of lower molars (viz., those parts which, when the jaws are at rest, are protruding over the corresponding parts of the opposite, jaw) have undergone the least modifications, that they are the more stable elements of molars. These same sides being generally more complex than the inner side above and the outer below, we may infer from it that the complex condition is the primary one, and that the reduced, simplified, state of the inner side above and the outer side below is a specialized condition, the beginning of which we see already in molars of Cretaceous Mammalia and in those of Ornithorhynchus. The extreme of this specialized condition is what has been called trituberculism, and considered to be a primitive pattern of Euthe-rian molars. It is not more primitive in Ungulata, Condylarthra, Creodonta, and Lemuroidea thau in Sciurinee, the species of which, when there is only one cusp on the inner side of upper molars, present an approach to trituberculism. Now, what is the meaning of this reduction on the inner side of superior, and the outer side of inferior molars ? W e have seen that in perfectly brachydont teeth the outer and inner sides of the molars present the least difference from each other in longitudinal extension, as well as in the number of their cusps; and that the superior molar becomes shortened on its inner side, as well as the inferior on its outer side, by the excessive development of some cusps (generally either one or three, rarely two, in Sciurinee) at the expense of others, which are present in such perfectly brachydont teeth as those of Eosciuri or Rhithrosciurus. The meaning of this process of reduction becomes obvious, when we consider that the internal cusps of superior, |