146 



EVOLUTION OF MAMMALIAN MOLAR TEETH 



with the evolution of the mammalian molar teetli in general to 

 suppose that the primitive Oligocene Sciurid of Fig. 105 actually 

 exhibits a tritubercular crown above without the hypocone, and a 

 (juadritubercular crown below, in which the paraconid is vestigial. 

 (Compare Fig. 106.) 



Among the rats and mice we observe a secondary cusp addition 



O i/ j. 



and evolution, which has obscured the primitive tritubercular pattern 

 it" such existed, closely parallel or analogous to that of the multi- 

 tubercnlates in the development of three parallel rows of tubercles 

 above and two parallel rows of tubercles below. (See pp. 102-104.) 



FIG. 105. Left upper cheek teeth of a primitive Squirrel, Sciurus (Prosciurus) rctustus from 

 the Titanotherium Beds (Lower Oligocene) of Montana, the molars showing apparent derivation 

 from the tritubercular pattern, x * After Matthew. 



Fio. 106. Lower jaw and teeth of Gyiiin<>/ii.>icl<n.< minimus, a primitive Sciuromorph (?) from 

 the Titanotherium Beds of Montana ; the molars apparently indicate derivation from the 

 tuberculo-sectorial type. MI seems to show a vestige of the paraconid. x A. After Matthew. 



FIG. 107. Upper teeth (crown view) of Mylarmulus monodon (Cf. Fig. 108), showing the small 

 size of in 1 , i/i'2, the great development of p$. In the rootless hypsodont teeth the enamel is 

 folded into longitudinal "lakes.'' All traces of a simpler crown pattern have been lost, as in 

 many other Hystricomorphs, Castoridse, etc. x jj. After Matthew. 



