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Show 1893.] DR. C. J. FORSYTH MAJOR ON MIOCENE SQUIRRELS. 207 a cardinal point and will, when once generally recognized, appear to be a simple truth. Bunodonty, as opposed to lophodonty, is the first step from a transverse arrangement towards the longitudinal one, and is not always to be distinguished at once from the second step I am speaking of, though this last is often characterized by a sort of asymmetry, or confusion, in a way, as is usually the case in transitional stages. First, as to Sciurinee. A s has already been said, transverse crests are to be found only in semi-hypsodont types, many of which tend towards the Hystricomorpha, which for their lack of brachydont molars at once show themselves to be more specialized forms. With the exception of Myoxine types-and this exception is only an apparent one-we may say that the more the molars tend towards brachydonty, the more the crests are broken up into cusps. Of these cusps there are generally five on the outer side of upper molars, two or three of which have been prominently developed. In the middle two intermediate, and on the inner side in the same manner as on the other side, originally a longitudinal series of cusps were developed, which very soon, viz., when the tooth ceases to be perfectly brachydont (as well as in somewhat worn semi-hypsodont or hypsodont teeth), are reduced in number and tend to become coalesced, a middle cusp appearing the most developed. This middle cusp seems to be for the greater part the remnant of a fifth series which have become partially atrophied, in order to give place for the median transverse valley; and it is in consequence of the formation of this valley that the cusps appear arranged in transverse series, even before being connected as ridges or crests. In superior and inferior molars, the most brachydont members of the family are at the same time those which show a tendency towards a longitudinal alignment of their cusps. The difference between superior and inferior molars consisting in the presence in upper molars of intermediate cusps, in more specialized, viz. less brachydont forms, generally reduced to two, as before stated, but which, as shown by the most brachydont forms, are the remnants of one or more longitudinal series of cusps or tubercles, intermediate between the outer and the inner series. The cup- or basin-like shape of inferior Sciuromorphine molars is but a slight specialization of a primitive type, a disposition of the cusps on the outer and inner margin, -with an intervening longitudinal depression. The slight specialization consists in the beginning of a transverse arrangement. In the Bornean Rhithro-sciurus (Plate IN. fig. 2), the whole of the very brachydont inferior molars consists mainly of two series of marginal cusps, none specially developed, and with a spacious longitudinal groove dividing the outer from the inner series; thus pointing significantly towards some primitive mammalian molars remote in time (Micro-lestes). And so the curious Pseudosciurus, from the Upper Eocene of Southern Germany, shows the tendency towards, or, as we rathe |