78 INVOLUTION OF MAMMALIAN MOLAR TEETH 



cular theory. The writer has recently made strenuous efforts to secure 

 additional evidence, which have not thus far been successful. In the 

 meantime too great emphasis cannot be laid upon the fact that all the 

 existing palcKontological evidence points in tl xttir <li ruction, namely, to the 

 presence of the chief cone upon the inner side of the upper molars, and 

 upon the outer side of the lower molars. An important oversight on the 

 part of those who are still unconvinced of the tritubercular theory, is the 

 necessity of a mechanical adaptation of the upper to the lower teeth 

 in every stage of development, which is perfectly met by the tritubercular 

 theory.* Given the universally acknowledged trigonid or triangular 

 arrangement of cusps in the lower teeth, no mechanical relations can be 

 imagined in an upper molar crown which originated with the external 

 cusps, paracone and metacone. 



If the main object of palseontological research is to trace back various 

 lines of descent as far as possible, the very unity of primitive type makes 

 this apparently more difficult than before, but not really so. We were 

 working before upon a false basis, or no basis at all ; we can now advance 

 upon the certain basis of primitive form and the one requisite of progress 

 is to employ much more exact methods of description and analysis. 



The Three Primary Forms. 



So far as the molar teeth were concerned, there were, to our present 

 knowledge, but three great primary forms, which succeeded each other as 

 stages and also persisted. From one or other of these all the known 

 recent or fossil mammalian teeth have diverged, including probably 

 the Multituberculates. These types are illustrated in the accompanying 

 cut. First, the liaplodont crown, which links the mammals with the 

 reptiles ; second, the triconodont crown which was predominant in the 

 Lower Jurassic period ; third, the tritnl'rcnlar crown which appeared in 

 the [Upper Jurassic or] Lower Cretaceous T and has been by far the 

 most productive. The transitions between these great types are found 

 among the Mesozoic mammalia and have already been worked out 

 with considerable care.t 



From each of these great primary stages it would at first appear that 

 some of the mammalia directly derived their dental type, for both the 

 " haplodont " and " triconodont " crowns are seen to-day among the 

 Cetacea. Yet there is ground for uncertainty here, for as the progressive 



* [See especially Cope, "On the Mechanical Causes of the Development of the Hard 

 Parts of the Mammalia," Jour. Morph., Vol. III., Sept. 1889, pp. 226-274. In opposition 

 see pp. 61*, 68*, 82*, Figs. 2U8, 209, of this volume. ED.] 



1 It now appears advisable that the so-called Como (Atlantosaurns) Beds of North 

 America and the Purbeck Beds of England should be placed in the base of the 

 Cretaceous instead of in the Upper Jurassic as formerly. [See p. 22*.] 



t [See, however, p. 222.] 



