CHAPTKK II. 



FIRST OUTLINE (1888) OF TRITUBERCULAR EVOLUTION 



IN MAMMALS. 



[Reprinted directly from the paper entitled "The Evolution of Mammalian Molars to 

 and from the Tritubercular Type, 1 ' 1 The American Xatiirali*t, December, 1888.] 



THE dentition in the recent Mammalia is so diverse that the most 

 sanguine evolutionist of fifteen years ago could not have anticipated 

 the discovery of a common type of molar, in both jaws, as universal 

 among the Mammalia of an early period as the pentadactyle foot, 

 and as central in its capacity for development into the widely 

 specialized recent types. 



The tritubercular molar, discovered by Professor Cope in the Puerco, 

 is exactly such a type, and may be considered with the pentadactyle 

 foot as playing a somewhat analogous role in mammalian history, with 

 this important difference the unmodified pentadactyle foot was pro- 

 bably inherited direct from the reptiles, and its subsequent evolution, 

 with a few exceptions, has been in the direction of the greater or 

 less reduction of primitive elements towards special adaptation, as, to 

 borrow an extreme illustration, in the transition from Plicnacoclm with 

 26 elements in the manus to Eym* with only 12 such elements. On 

 the other hand, the tritubercular tooth was not inherited, but in all 

 probability developed within the mammalian stock, from a hypothetical 

 form with almost, if not quite simple conical molars, implanted by single 

 fangs, in a nearly homodont series. 2 No such primitive type of mamma- 

 lian dentition is actually known, although Dromatherium approximates it ; 

 but the apparent reversion to this type among the Cctacca, and apparent 



J Read in the geological section of the British Association at Bath, September, 1888. 

 Read in abstract by Professor Cope, National Academy of Sciences, at New Haven, 

 Nov., 1888. 



2 See Author, "Structure and Classification of the Mesozoic Mammalia," Jour. 

 Phila. Academy, 1888, p. 240. 



