98 EVOLUTION OF MAMMALIAN MOLAR TEETH 



5. BASAL EOCENE MAMMALS. 



In the basal Eocene, Puerco, Torrejon, and Fort Union formations of 

 North America, we again find the mammals divided into multituberculates 

 and trituberculates. 



Among the latter the law of trituberculy was first perceived by Cope ; 

 they include animals possibly related to the Primates and Rodentia, also 

 ancestral Edentata (?), Creodonta ancestral to the Garni vora, Condylarthra 

 collateral ancestors of the Perissodactyla and Artiodactyla, and Ambly- 

 poda, a very primitive order of Ungulates totally distinguished from all 

 others by the fact that the teeth evolve throughout on the triangular or 

 tritubercular basis. 



The American Museum collection as revised by Dr. W. D. Matthew, 

 omitting the Multituberculates, contains 1200 identified specimens, 

 referred to 62 species, 33 genera, 6 orders. All flic upper molar* of these 

 animals without exception are either : (].} purely tritubercular or triangular, 

 or (2) transitional to the quadritubercular and quadrate form Tjy the addition 

 of a hypocone. This more extended knowledge of the basal Eocene not 

 only reinforces the evidence originally advanced by Cope, but affords an 

 almost overwhelming body of proof as to the fundamentally trigonal 

 tritubercular character of the upper molars of these orders. 



The molar teeth exhibit a marked progression upon those of the Upper 

 Cretaceous, in three very important characters: (1) there is an internal 

 cingulum in a very large percentage of the upper molar teeth, (2) a 

 hypocone or postero-internal cusp is developing from this cingulum in a 

 somewhat smaller percentage of upper molars : (3) the paraconid or 

 antero-internal cusp of the lower molars is disappearing in some of the 

 lower molars. 



Numerically, the most progressive upper molars present three fangs, 

 six cusps : namely, protocone, paracone, metacone, hypocone, protoconule, 

 metaconule, and two styles, parastyle, mesostyle, and metastyle. The 

 lower molars present also six cusps, protoconid, paracouid, metaconid, 

 hypoconid, hypoconulid and entoconid, the latter three arising from the 

 talonid or heel. 



6. LOWER EOCENE MAMMALS. 



In the lower Eocene, Suessonien of Europe, Wasatch of North America, 

 the majority of modern groups of mammals are represented, including the 

 following orders : Primates, llodentia, Insectivora, Creodonta, Carnivora, 

 Tillodontia, Edentata, Condylarthra, Amblypoda, Perissodactyla, Artio- 

 dactyla. 



All of these animals are either tritubercular or in stages evolving from 



