206 EVOLUTION OF MAMMALIAN MOLAR TEETH 



very generalized type, precisely such as we anticipate to meet with in 

 a refuge for old and little modified forms." 



The first conclusion drawn is that in the most lirachyodont Sciurine 

 molars the crowns are quadrate, and the cusps tend toward longitudinal 

 arrangement with two entire outer and inner ' marginal series ' of 

 cusps in the lower molars, and with two ' marginal ' and a more or less 

 complete ' intermediate series ' in the upper molars. From this it is 

 inferred (p. 205) that the nmlticuspidate or polybunous condition is the 

 most primitive, and that the triangular or tritubercular condition repre- 

 sents an extreme of specialization. 



Analogous arguments are applied by this author to other mammals. 

 The polybunous molar of ^-Elurus is regarded as one of the most primitive 

 among the Carnivora, the polybunous Ardocyon is regarded as the most 

 primitive among the Oreoclonts. By similar reasoning we should consider 

 the polybunous tuberculate molars of the bear and of the orang as 

 respectively more primitive than the high pointedly cusped molars of 

 the typical Viverrines (cf. Fig. 102), or of the lower Primates 

 (see pp. 158-160). The author concedes (p. 185), that in certain bats 

 (see p. 129), and in Cheiromi/s the basin-shaped molar is retrogressive 

 on account of the secondary assumption of fruit-eating habits ; but regards 

 (p. 213) the basin-shaped polybunous molar of J/iV/Wr.sA'.s as primitive 

 and ancestral both to the multituberculate and trituberculate types. 



By similar reasoning he finally reaches the conclusion that in the 

 primitive molars of placentals the cusps were arranged in longitudinal 

 rows, three rows with two intermediate grooves in the upper teeth, and 

 two rows with one intermediate groove in the lower teeth ; in other 

 words, that the ancestral type of Eutherian molar was multituberculate. 



Dr. A. Smith Woodward (Vert. Pal, p. 269) feels the force of this 

 argument as well as that of the embryologists, and concludes. " Hence 

 this at first sight brilliant generalization [of primitive trituberculy] 

 can only be accepted at present as a convenient working hypothesis 

 which remains on its trial." 



Mr. E. S. Goodrich 1 in his discussion of the Lower Jurassic mammalia, 

 after enumerating several gaps in the argument for the tritubercular 

 theory, concludes: "The common ancestor of the tritubercular sectorial 

 mammals and of the Multituberculates probably had teeth of an indefinite 

 multituberculate pattern, which gave rise, on the one hand, to elaborate 

 multituberculate teeth, and on the other to the tritubercular sectorial. 

 Thus the development of two longitudinal rows of three cusps would give 

 rise to the type of lower molar common amongst the Multituberculata ; 

 the fusion of the two anterior of these cusps or the loss of one would 



1 " On the Fossil Mammalia of the Stonesfield Slate," Quar. Jour. Mirr. Sci., Vol. 

 XXXV., 1894, pp. 407-432. 



