Marsh Collection, Peabody Museum. 43 



small posterior intermediate cusp is also developed. The pat- 

 tern of the crown of the first molar is almost an exact coun- 

 terpart of that of the fourth premolar, with the exception 

 that the postero-external cusp has undergone still further 

 enlargement, and the internal cusp has increased in size until 

 the tooth crown is now nearly triangular. The anterior cusp 

 of the two externals is still distinctly the larger, and has a 

 position well towards the middle of the outer border. The 

 posterior intermediate cusp is not developed. The second 

 molar is interesting as showing a stage of development inter- 

 mediate between that of the first molar and the fourth pre- 

 molar. The internal cusp is much enlarged, but the postero- 

 external is yet small. 



Having thus described at some length the first term of the 

 series, let us now turn to the last or final stage, to discover, if 

 possible, just what changes have been effected. It may be 

 added, en passant, that every intermediate stage between the 

 two is known, and that, moreover, from strata of intermediate 

 position with respect to the time scale. In Mesonyx, figure 

 64, we observe that very little change has taken place in the 

 first and second premolars. In the third, the internal cusp has 

 disappeared and the posterior has been enlarged. In the 

 fourth premolar the postero-external cusp has increased in 

 size until the two externals are equal, and the internal cusp is 

 enlarged so as to give a perfectly molariform type of crown. 

 The position of the principal or primitive cusp is now more 

 or less at the antero-external angle on account of the increase 

 in the size of the postero-external, as well as the growth of 

 the internal cusp. The crowns of the first and second molars 

 have become fully tritubercular, the two external cusps are of 

 equal dimensions, and are situated more or less exactly at the 

 two external angles. The last molar has completely disap- 

 peared. 



So much for the facts. Let us next direct attention to their 

 application and general bearing upon the determination of the 

 order of addition and homologies of the respective elements 

 composing the mammalian tritubercular molar crown. I shall 

 purposely limit what I have to say here upon the subject to a 

 consideration of the superior molars, and I shall, moreover, 

 confine my remarks as nearly as possible to the present 

 phylum, for the reason that it offers the best and most direct 

 evidence with which I am acquainted, and by means of which 

 an understanding of some of the primary truths of cusp 

 addition in the true molars of the placental Mammalia' can be 

 had. It must be remembered, however, that much evidence 

 of a similar character in other phyletic series whose history is 

 quite as well established is not altogether wanting. 



