EVIDENCE BEARING ON TOOTH-CUSP DEVELOPMENT IOI 



tive trituberculate forms and in all forms derived from a tritu- 

 bercular type of tooth as well, except where the main inner cone 

 (protocone) has been reduced secondarily. The opposite view 

 held by the tritubercular 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 fur- 

 nish 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 Dryolcstes and 

 Paurodon we have trituberculate 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 anlero-internal cusp (para- 

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

 The last two form the base of the trigonid. In Dryolcstes both 

 the trigonid and the pimitive heel are somewhat more advanced 

 in development. In still other forms, such as Manacodon and 

 Tinodon, the two internal cusps are relatively large and the 

 trigonid is fullv 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 

 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 pos- 

 terior 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. 



'This is in accord svith the general conclusions on tooth cusp development 

 reached bv Herluf Winge as early as 1882. Widinsk Meddelelsor fn den natur- 

 hist. Florening e Kjobenhavn. iSS;. p. iS. 



