80 EVOLUTION OF MAMMALIAN MOLAR TEETH 



that many features of primitive form may be lost ; they may quite 

 as readily be interpreted as tritubercular as multitubercular, especially in 

 the embryonic stage as described by Poulton. 



It is not difficult however to establish the principle that a true multi- 

 tubercular tooth may spring from a tritubercular tooth. As pointed out 

 elsewhere, my friend, Prof. J. A. Allen, directed my attention to the 

 " multituberculate " rodents. A comparison of Mas, Dipodomys and 

 Perognathus beautifully illustrates the stages between " trituberculy " 

 and " multituberculy " in living types. The three rows containing twelve 

 tubercles in the latter genus are derived respectively from the " external," 

 " intermediate " and " internal " cusps of a sexitubercular bunodont type 

 similar to the Hyracothcrium molar on a small scale. The additional 

 cusps are successively added to each row. Thus the upper molar of 

 Perog natlius is closely analogous to that of the Mesozoic Multituberculata, 

 especially to such a type as Tritylodon. Passing also from the higher 

 Multituberculata to the lower and more ancient, we find fewer and fewer 

 cusps until we reach a " paucitubercular " parent form in the Upper 

 Triassic Microlcstes. Microlestes itself was not tritubercular ; it had 

 a basin-shaped crown surrounded by irregular tubercles ; this basin, 

 however, was not dissimilar to that in molars of the Eocene rodent 

 Plesiarctomys which is obviously of tritubercular origin.* 



This evidence has been recently reinforced in a most striking manner 

 by the discoveries of Professor Seeley in the Karoo Beds of South Africa, 

 from which two principal conclusions may be derived : First, that Tritylodon, 

 formerly placed with the mammalia, contains a large number of reptilian 

 characters. Since the fossil is closely related on the other hand to 

 the remaining Multituberculata, it appears possible that we have in the 

 Gomplwdontia the group from which the Multituberculates sprang. 

 A study of the dentition of other Theriodonts in the Karoo Beds shows 

 that while Tritylodon and Trirachodon are typically Multituberculates, 

 others, such as Diademodon t have a trituberculate pattern, exactly such a 

 pattern as we find in certain Lower Eocene mammals. Altogether there 

 is certainly increasing support for the writer's hypothesis, that the 

 multituberculate tooth is of tritubercular origin. 



The Early Stages of Sexituberculy. 



The Tr if/on. Kespect for Cope's priority should not prevent our 

 ultimately adopting the late Professor Kutimeyer's term trigonodont for 



* [The derivation of some multituberculate types from trituberculate types does Dot 

 prove that all multituberculate types have been derived from trituberculate types, and 

 reasons are presented on p. 105*" for thinking that the ancestral multituberculate molar 

 as represented in Mirrolestfis was not derived from a typical tuberculo-sectorial lower 

 molar. ED.] 



tfSee p. 92.] 



