448 II. F. Osborn — Mammalia in North A 



merica. 



Art. LXI. — The Rise of the Mammalia in North America ; 

 by Henry Fairfield Osborn, Columbia College. New 

 York. 



[Continued from page 392.] 



Part II. 



Origin and Evolution of Tritiiberculism. 



" Concrescence " is the newest theory of cusp evolution — 

 an expansion by Kiikenthal and Rose of views earlier ex- 

 pressed by Gaudry, Magitot and Dybowski. As Kiikenthal 

 derives three conical Cetacean teeth by splitting apart a trico- 

 nodont molar, he conversely derives a triconodont molar by 

 bringing together three reptile cones. Smith Woodward has 

 called attention to the support the epidermal structures of 

 the fishes give to this hypothesis, yet as applied to mammalian 

 teeth, it comes from a one-sided Morphology which regards 

 only the wonderful though mutilated chapters of Embryology 

 when the untom pages of Paleontology are at hand. Between 

 the Trias and the Puerco, we are, so to speak, in at the birth 

 of every successive cusp, and can observe positively that the 

 law of cusp evolution is direct upgrowth from the smooth 

 slopes of the crown or from the cingulum, that fertile parent 

 of new cusps. Each new cusp is usually preceded by an 

 abraded surface, and prophesied by an excessively minute hil- 

 lock. It follows from this that cusps range in size and height 

 directly according to their age — a principle beautifully demon- 

 strated in some of the Mesozoic teeth. If the Kiikenthal- 

 Rose theory were correct, the oldest triconodonts should be 

 iso-conid, whereas we know that the three equal cones of Tri- 

 conodon are all a very late development ; the earlier forms 

 show the lateral cones receding to the needle-points of Dro- 

 matherium. 



The tritubercular molar owes its survival to the original 

 advantage of its triangular form, and to the possibilities of free 

 cusp addition — as worked out by Cope, Wortman, Schlosser, 

 Scott, and myself. Riitimeyer's term, " trigonodont," best 

 expresses the primitive structure of the upper and lower teeth, 

 as of two interlocking triangles with their open bases turned 

 outward in the upper and inward in the lower jaw. These 

 " trigons " cutting past each other, made a shear so perfect 

 that many Insectivora retained it without further evolution. 

 But in most Trituberculates a talon was next added to the 

 lower molar (Jurassic stage) as a pestle crushing into the upper 

 valley ; this talon gradually widened into a broad heel sup- 



