32 EVOLUTION OF MAMMALIAN MOLAR TEETH 



gradual increase of size and prominence of the lateral cones until they 

 are upon the level of the main cone and sub-equal to it, this increase 

 being accompanied by a marked elongation of the crown so that the three 

 molars of Triconodon occupy a greater proportion of the jaw than is taken 

 by the seven molars of Dromatherium. This unmodified triconodont type 

 is very rare in the more recent mammalia. It persists in the lower jaw, at 

 least, of Dissacus from the Puerco, and in the lower molars of Thylaciiin*, 

 the upper molars presenting an internal heel.* 



(&) In his paper upon the Creodonta, 1 Cope observed that the Spala- 

 cotJierium molars (Figs. 4, 11) represents a stage of transition between the 

 triconodont and tritubercular molars. There can be no doubt that the 

 cusps seen upon the inner face of the inferior molars of this genus are 

 homologous with the para- and meta-cones,t and there are several facts 

 which support Cope's hypothesis that they represent a stage of inward 

 rotation of cusps which were at an earlier stage in the same fore and aft 

 line with the main cusp. These are, that in Phascolotherium (Figs. 4, 6) 

 the lateral cones are seen to be slightly internal to the main cone so that 

 their median slopes descend upon the inner face; in Tinodon (Fig. 10), 

 of a later geological period, this position is slightly more pronounced ; in 

 Menacodon (Fig. 9) it is still more marked, but less so than in 

 Spalacothenum (Fig. 11). These genera, although evidently in two 

 different lines of descent, afford the desired transition stages. The 

 Spalacotlierium molar as seen from above 2 has a striking resemblance to 

 the anterior sectorial triangle of the Stypolophus or Didymictis molar of 

 the Puerco. It is in fact sub-triangular, the superior molars probably 

 having the lateral cones rotated outwards, so that the upper and lower 

 molars form an alternating series, the ridges connecting the main and 

 lateral cones acting as sectorial blades. 



The question now arises whether the Stylacodon- (Fig. 29) molar 

 represents the next higher stage of development, viz., the tubercular- 

 sectorial molar in which the anterior triangle is followed by a low heel. 

 And if so has the Stylacodon type passed through the stages of inward 

 rotation of the lateral cusps ? The superior aspect of the Stylacodon 

 molar presents an anterior triangle with the long styloid cone forming the 

 apex and connected by divergent ridges with the anterior pair of cusps ; 

 behind these is a third cusp not connected by a ridge with the styloid 

 cone. In the upper jaw the three cusps are external and the single cone 

 internal, these relations are reversed in the lower jaw. We cannot well 



* [This condition is now believed to be not primitive but secondary in both the genera 

 mentioned. Eu.] 



1 " The Creodonta," American Naturalist, 1884, p. 259. 



t[But see page 33* where the homologies of these cusps with the paraconid and meta- 

 conid of Trituberculates are doubted. ED.] 



2 Owen, The Mexozoic Mammalia, Plate I., Fig. 32c. 



