182 DE. r. J. roRSTTir ^fajor ox isiiock>e squirrels. [Feb. 28, 



types before mentioned — the Seiunis vulgaris and the Xems types, 

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



*yp*^' .... 



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

 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 w hich have an almost equal longitudinal extension ; and 

 a minimum of transverse ai'rangement of their cusps, which sho\v 

 a tendency towards a longitudinal disposition. 



2. In the upper molars of the Se.-vnlr/ans-tyi^e a transxerse 

 arrangement is already conspicuous. The cusps have parfly 

 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 intervening 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. vuhjaris, e. g.), the inner side of 

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

 pai'tite division, the median cusp being the stoutest. Still more 

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

 Oriental Squirrels, which in other characters (of the skull &c.) 

 as well as in the dentition approach Sc. vtihjarls. The molars, 

 ho\Aever, are somewhat .stouter, and the cusps and ridges more 

 prominent. In unw orn teeth of Oriental forms, e. (/. of Sc. pre- 

 vosti (Plate YIII. fig. 2) or Sc. lolroides (Plate VIII. fig. 3), 

 the inner margin is rather elongate, and shows more distinctly 

 than does ^V. vvhjans the tripartite division with a prominent 

 medial cusp. When the teeth have become somewhat worn, 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 8c. vvlgaris, 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 tlieir inner side) appears to be a general and 

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

 among Cretaceous Mammaha, as well as in the recent Onritho- 

 rJu/nchns. As to its meaning, we shall have to consider it 

 afterwards. 



In lower molars of the >Scwras-vulfjaius-type, two cusps, thi; 

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

 trausversely. Anteriorly to these we have a small transverse 

 valley, bordered in front by a transverse ridge, which is more 

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

 is generally one cusp only in front. 



The hinder ];art of inferior molars is shaped into a sort of cup, 

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



