EVOLUTION OF MAMMALIAN MOLAR TEETH 



and in all forms derived from a tri tubercular type of tooth as well, 

 except where the main inner cone (protocone) has been reduced 

 secondarily. The opposite view held by the tri tubercular theory now 

 apparently stands on very insufficient evidence, and the proposition that 

 the protocone, of Osborn, represents the primary cusp is entirely without 

 support. 



' The lower molars of the Atlantosaurus beds mammals furnish 

 abundant additional evidence along the line of conclusions regarding 

 the shifting of three cusps from a straight line to form the primitive 

 triangle. In such forms as Dri/olcstes and Paurodon \\Q have trituber- 

 culate molars in the primitive or forming stage, and, what is most 

 significant, the cusps resemble very closely, both in position and relative 

 proportions, those of the premolars of later types in their early stages 

 of transition to the molariform pattern. In the lower molars of 

 Paurodon the crown consists of a high, pointed cusp (protoconid), 

 centrally placed, a low posterior heel, a small antero-internal cusp 

 (paraconid), and a very small median internal cusp (metaconid). The 

 last two form the base of the trigonid. In Dryolestes both the trigonid 

 and the pimitive heel are somewhat more advanced in development. 

 In still other forms, such as Mcnacodon and Tinodon, the two internal 

 cusps are relatively large and the trigonid is fully developed, while the 

 heel, or talonid, is very small or entirely wanting. In all the paraconid 

 and metaconid are entirely on the internal side of the crown, and in 

 these and all the material examined there is not the slightest evidence 



O 



of any shifting of the cusps, but they seem to have arisen in the 

 positions they now occupy. 1 In Paurodon the heel is apparently as 

 much or more developed than either of the internal cusps and seems 

 to have made its appearance even in advance of the metaconid. Also 

 the metaconid is still very rudimentary and is just budding off near 

 the base of the protoconid, but little posterior to its apex and midway 

 of the entire length of the crown, while the place of origin assigned 

 to it by the tritubercular hypothesis is already occupied by the 

 comparatively large heel. 



' From these observations it seems apparent that the trigonid of 

 the lower molars is not the reverse of the trigon of the upper molars, 

 as held by advocates of the tritubercular theory, and the homologues 

 of the elements of the upper and lower -molars, as proposed by this 

 theory, are far from being apparent. (This also accords with the 

 conclusions of Winge.) 



' The lower molars of Triconodon differ from any of the forms just 



1 This is in accord with the general conclusions on tooth cusp development reached 

 by Herluf Winge as early as 1882. Vidensk Meddelelscr fra den naturhist. Forening 

 i Kjobenhavn, 1882, p. 18. 



