366 Wo7'tma7i — Studies of Eocene Mammalia in the 



the Tertiary are known. In the earliest Eocene, the molars, 

 with a few notable exceptions, had already assumed such a 

 de£;ree of complexity as practically to obliterate all traces of 

 the order of appearance and manner of development of the 

 cusps. When, however, the ancestors of the Puerco fauna are 

 found, and the more primitive stages of their tooth develop- 

 ment obtained, 1 look forward with the utmost confidence to 

 the production of all the evidence necessary to a complete and 

 final demonstration of the truth of this hypothesis. 



While this principle, enunciated by Scott, may be made to 

 include any given group of mammals, and the history of their 

 molar cusps thus determined, yet at the same time I feel well 

 assured that no general law can be framed nor can any termi- 

 nology be devised which will be applicable to all the Mammalia, 

 unless it is confined strictly to the position of the cusps, with- 

 out any reference whatever to their homologies. The reason 

 for this difficulty is, that different groups of mammals have 

 adopted different plans for increasing the complexity of their 

 molars. In many divisions, the order of appearance and posi- 

 tion of the cusps, as outlined by Scott, undoubtedly obtains ; 

 but in others, as I shall presently show, it has been different. 



Taking as a starting point a transversely flattened conical 

 crown, a complicating premolar of the inferior series, in a large 

 number of groups of the Mammalia, passes through the follow- 

 ing stages : (1) The posterior edge or slope of the crown 

 elongates and develops a second or posterior root ; (2) this 

 slope of the crown becomes thickened transversely, and flat- 

 tened from before backward, so as to present a triangular area 

 with the apex at the summit ; (3) this area looks upward and 

 backward, and is bounded by a descending ridge on each side ; 

 (4) a thickened ledge is formed at its base, foreshadowing the 

 heel ; (5) on the inner descending ridge, bordering the posterior 

 triangular area, appears a new cusp, small at first, which is 

 jposterior and internal to the main cnsjp ; (6) concomitantly, 

 the heel broadens and its posterior edges grow up in such a 

 way as to form a basin ; (Y) at the same time a cusp may or 

 may not be developed, at the anterior slope of the crown ; (8) 

 the heel develops two cusps, one of which is external and one 

 internal in position. 



Thus, it will be seen that all the elements necessary to the 

 formation either of the quadritubercular or of the so-called 

 tuberculo-sectorial crown are present, and further growth of 

 the new elements is all that is required to effect a complete 

 molariform transformation. That the evolution and develop- 

 ment of certain premolars has taken place in this manner, is 

 8U])ported by a great abundance of evidence from many well- 

 known phyla whose history has been determined with consid- 



