Evolution of Mammalian Molars. 1067 
THE EVOLUTION OF MAMMALIAN MOLARS TO 
AND FROM THE TRITUBERCULAR TYPE. 
BY HENRY FAIRFIELD OSBORN. 
HE dentition in the recent Mammalia is so diverse that the most 
sanguine evolutionist of fifteen years ago could not have anti- 
cipated the discovery of a common type of molar, in both jaws, as 
universal among the Mmamalia of an early period as the penta- 
dactyle 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 rôle in mamma- 
lian history, with this important difference—the unmodified penta- 
dactyle foot was probably inherited direct from the reptiles, and its 
subsequent evolution, with a few exceptions, has been in the direc- 
tion of the greater or less reduction of primitive elements towards 
special adaptation, as, to borrow an extreme illustration, in the 
transition from Phenacodus with 26 elements in the manus to 
Equus with only 12 such elements. On the other hand, the tritu- 
bercular 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.? No such primitive type of mammalian 
dentition is actually known, although Dromotherium approximates 
it; but the apparent reversion to this type among the Cetacea, and 
apparent retention of it in the Edentata,’ support all the independent 
evidence upon this point derived from the Mesozoic 
The principle of growth was the regular addition of new parts to 
the simple cone, not at random, but according to a certain definite 
' Read in the geological section of the British Association at Bath, 
September, 1888, Read in abstract by Prof. Cope, National Academy 
is Sciences, at New Haven, Nov., 1888. 
* See Author “ Structure and Classification of the Mesozoic Mamma- 
lia.” Jour. Phila, Academy, 1888, p. 240. 
* See Oldfield Thomas, “ The Homologies and Succession of the Teeth 
in the Dasyuridw.” Phil. Trans., 1887, p. 458. 
