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1893.] DE. C. S. rOESITH MAJOB. OS illOCKSE SyUlEEHLS. 20/ 



a cardinal point aud will, wheu ouce geuei-ally recognized, appear 

 to be a simple truth. 



Bunodontv, 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 1 am 

 speaking of, though this last is often characterized by a sort of 

 asymmetry, or confusion, in a \\a,x, as is usually the case in 

 transitional stages. 



First, as to Sciurince. As has already been said, transverse 

 i-rcsts are to be found only in semi-hypsodont ty})es, many of which 

 tend towards the Hystricomorpha, which for their lack of bruchy- 

 dont molars at once show themselves to be more specialized forms. 

 With the exception of Myoxine types — -and this exception is oidy 

 an apparent one — we may say that the more the molars tend 

 towards brachydonty, the more the crests are broken u]) 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, aud on the inner side in the same 

 manner as on the other side, originally a lomjitudinal series of 

 cusps were developed, which very soon, viz.,Avhen the tooth ceases 

 to be perfectly brachydont ('as well as in somewhat worn semi- 

 hypsodont or hypsodont teeth ), ai-e reduced in number aud 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 ha^■e become partially atrophied, in order to gi\ e 

 place for the median trans\erse 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 a\ liich show a tendency 

 towards a longitudinal alignment of their cusps. The diiierence 

 between superior and inferior molars consisting in the presence in 

 upper molars of intermediate cusps, in more specialized, \\z. less 

 brachydont forms, generally reduced to two, as before stated, but 

 which, as shovATi by the most brachydont forms, are the remnants 

 of one or more longitudinal series of cusps or tubercles, inter- 

 mediate 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 lUdtltro- 

 sciunis (Plate IX. fig. 2), the whole of the Aery brachydont inferior 

 molars consists mainly of two seines of marginal cusps, none 

 specially developed, and with a spacious longitudinal gi-oove dividing 

 the outer from the inner series; thus pointing significantly to- 

 wards some primitive mammalian molars remote in time {Micro- 

 lestes). And so the curious F.feudosciurm, from tlie Upper Eocene 

 of Southern Germany, shows tbe tendency towards, or, as we rathe 



