TKKTH OF TUK MESOZnlC MAMMALS 



avoid the inference that the M,//<ir<,t/t,n lower molar is a specialized 

 tubercular-sectorialj that the styloid external cusp, which until Marsh's 

 discovery of Drii<>l<-*ti'* (Figs. 31-34) was rognnlrd as the single summit 

 of the crown, is the protocone while the anterior pair of internal cusps 

 represent the paracone and metacone, followed by a third element, the 

 hypocone or heel. This is further confirmed l>y the transition to the 

 simpler Spalacotherium type seen in the molars of Asthcmnlnn (Fig. 35) 

 in which the hypocone is entirely wanting while the remainder of the 

 crown is closely similar to that of Stylacodon. The internal cusps 

 present many degrees of development in different members of the 

 Stylacodontidse ; in Lna<lon (Fig. 30) they are much less prominent than 

 in T)/'//olcstes, the heel being also inconspicuous. While the relations of 

 the four cones composing the Stylacodont crown strongly suggest the 

 tubercular-sectorial molar there is one matter of doubt in the way of 

 the derivation of this tooth from the Spalacotherium type (Fig. 11); 

 that is, the position of the fangs. In Spalacotherium and Menacodon 

 (Fig. 9) the fangs are paired and placed beneath the para- and 

 metacones. In the Stylacodonts the external fang is directly beneath 

 the protocone ; the question is, does this represent the anterior or 

 posterior, or an additional fang ? 



4. The molars which have been considered thus far show directly 

 or indirectly the triconodont type, i.e., the presence at some stage 

 of their evolution of the central and two lateral cones. In the 

 Amphitheriidfe (Figs. 15-21) it is clear that the main cone and 

 the lesser one, upon its anterior slope, represent the protocone 

 and paracone but it is uncertain whether the basal cusp, seen for 

 example upon the external face of the Diplocynodon molar (Fig. 20) 

 is homologous with the metacone or hypocone. The latter alterna- 

 tive excludes the development of the metacone or the passage 

 of these genera through a triconodont stage, and implies a con- 

 siderable separation of the Amphitheriidse from the stem of the 

 two families already considered. The former involves the supposition 

 that the metacone has metamorphosed into a heel.* The most primitive 

 molar in this family is seen in Eitnt'ixlon 1 (Fig. 19). The crown has 

 an obtuse recurved protocone, more like that of a premolar ; upon 

 the anterior slope is a rudimentary paracone which affords the only 

 means of distinguishing the molars from the premolars. The posterioi 

 slope terminates in a low extended heel. This molar pattern largely 



* [The questions and difficulties here stated are largely resolved by the hypothesis that 

 in the Triconodontidce the so called para- and metaconids are direct outgrowths from the 

 molar crown, while in the Amphitheriidse and Amblotheriidse the para-, meta- and 

 hypoconids have been derived from the internal cingulum. See page 8. ED.] 



1 Marsh, "American Jurassic Mammals," Am. Jour. Sc., April, 1887, PI. X. 

 Fig. 4. 



C 



