182 DE. C. J. rOESTTII MAJOR OK MIOCEKE SQriKBELS. [Feb. 28, 



types before meutioned — the Schtrvs vuhjarls and the Xenis tj^pes, 

 as we may call them — are each of them derived from a bracbydont 

 type. 



1. Therefore, beginning with the type of the most bracbydont 

 Sciurine molar, as being the most generalized, the various forms 

 belonging to it have a very flat elongate crown, the inner and outer 

 sides of which have an almost equal longitudinal extension ; and 

 a minimnm of trans^-erse arrangement of their cusps, which show 

 a tendency to\\ards a longitudinal disposition. 



2. In the upper molai-s of the Sc.-culr/aris-type a transverse 

 arrangement is already conspicuous. The cusps have partly 

 united to form transverse ridges, so that we see here the beginning 

 of a transition from bunodontism into lophodontism : four more 

 or less transverse ridges, the two median being the stoutest, with 

 three inter^■ening valleys. On the outer side are three prominent 

 cusps, corresponding to the three anterior ridges. A characteristic 

 feature on the inner side of the upper molars is an apparently 

 single cusp, which fits into the cup- or basin-shaped hollow of the 

 inferior molar, somewhat like a pestle in a mortar. In examining, 

 however, quite unworn teeth (of Sc. ruh/aris, e. g.), the inner side of 

 the upper molar presents itself more elongate and shows a tri- 

 partite division, the median cusp being the stoutest. Still more 

 is this seen in the upper molars of most of the middle-sized 

 Oi'iental Squirrels, which in other characters (of the skull &c.) 

 as well as in the dentition approach Sc. vuh/aris. The molars, 

 however, are some^-hat stouter, and the cusps and ridges more 

 prominent. In unworn teeth of Oriental forms, e. g. of Se. pre- 

 vosti (Plate YIII. fig. 2) or Sc. hhroides (Plate Till. fig. 3), 

 the inner margin is rather elongate, and shows more distinctly 

 than does *SV. vulgaris the tripai'tite division with a prominent 

 medial cusp. When the teeth have become somewhat «orn, these 

 divisions tend to disappear ; so that the usual aspect of worn 

 upper teeth in these Oriental Squirrels is that presented by the 

 somewhat worn teeth of /SV. vulgaris, viz., a single broad internal 

 cusp. The shortening, or, as one might say, the reduction and 

 simplification of the inner side of the upper molars compared to 

 the outer side (and, as may be added, of the outer side of inferior 

 molars compared to their inner side) appears to be a general and 

 primitive tendency of molar teeth ; in fact, we meet with it already 

 among Cretaceous MammaUa, as well as in the recent Omhlio- 

 rJiyncJms. As to its meaning, we shall have to consider it 

 afterwards. 



In lower molars of the Scu(n(s-vulgaris-ij]^e, two cusps, the 

 antero-external and the antero-internal, show a tendency to unite 

 transversely. Anteriorly to these we haAe a small transA-erse 

 valley, bordered in front by a trans^■erse ridge, which is more 

 rarely (Sc. pahnannn) raised in two cusps. In the premolar there 

 is generally one cusp only in front. 



The hinder jjart of inferior molars is shaped into a sort of cup, 

 to receive, as above mentioned, the internal tubercle of the upper 



