122 



EVOLUTION OF MAMMALIAN MOLAR TEETH 



in Figs. 64-67, that the hypocone is secondarily developed. Moreover, 

 even in the quadritubercular Hylomys, Necrogymnurus, and G-alerix, 

 the third upper molar is trituhercular. 1 



The remote ancestors of the Zalambdodonta or more primitive division, 

 with trigonal or triangular molars, have already been described under 

 the " Insectivora Primitiva" (pp. 22-30). Under the modern repre- 

 sentatives of this primitive type have usually been included such forms 

 as Ccntctcs, Soleuodon, and Chrysochloris (Figs. 68, 69). More recently, 

 on embryological grounds (pp. 209-213), this comparison has been 

 seriously questioned, and there are a number of authorities who believe 

 that the homologies proposed between the cusps in the recent Insectivores 

 and in the Jurassic Insectivores are unfounded. Our most recent 

 discoveries, however, seem to lend fresh support to the older view ; 

 these discoveries include a lower Oligocene Insectivore fauna from 

 Montana, fully described and figured by Matthew. 2 Among these 

 animals we especially note Apternodus (Fig. 70), in which the 

 lower teeth strikingly resemble those of Amphitheriiim ; and Micro- 

 pternodus, in which the lower molars resemble those of Solcnodon. 

 Again the teeth of Ictops tJwmsoni (Fig. 66) remind us of the upper 

 teeth of Pkascolestes (Uryolesfcs, Fig. 14). A higher stage of evolution 

 is represented by the lower and upper teeth of Ictops acutidens 



pr^/yc* 



A 



D 



FIG. 74. Tuberculosectorial and cainassial lower molars in a Viverrid and Insectivores. 

 A. Euptires govdoti, a very primitive Viverrid, resembling an Insectivore (crown view of /,). 

 x }. B. Cr,,t,tes ccaudaius (crown view of MJ), representing the ancestral form of C. xf. 

 (', D. Carnassial modification of m 2 in an Insectivore, Hemiccntetes mculago.scariensis. C. Crown 

 view. X T . D. Inner side view. XT- Note the lateral compression of the tooth, enlargement 

 of pt-'i, pud, reduction of med, tuld, as in the carnassial of Carnivora (Fig. 94, D). All from Forsyth 

 Major, Philos. Trans., Vol. 185 (18<>4), B. p. 24. 



(Fig. 67). Moreover, the teeth of Ericulus setosiis (Fig. 72) suggest 

 those of the Upper Jurassic (or Basal Cretaceous) genus Kvrtodovi 

 (Fig. 13). 



From another locality and from a more recent horizon in the 

 American Oligocene is the form Proscalops mioca-mis 3 (Fig. 73), a 

 primitive mole with rather simple trigonal molars, in which, however, 



1 Matthew, W. D., Bull. Amer. j)/.v. Nat. Hist., Vol. XIX., 1903, p. 228. 



2 " The Fauna of the Titanotherium Beds at Pipe stone Springs, Montana," Bull. Amer. 

 Mus. Nat, Hist., Vol. XIX., 1903. 



3 Matthew, W. D., Mtm. Amer. Mm. Nat. Hist., Vol. I., Pt. VII., 1901. 



