SUMMARY 523 



are three-toed, with progressively long-crowned teeth. All Pleistocene 

 horses are one-toed, with very long-crowned teeth. The Pliocene horses 

 are less perfectly known, but intermediate in type. But these are broad 

 generalities. Ten faunal zones are readily distinguished in the American 

 Tertiaries, each characterized by the first appearance of a distinct generic 

 stage of Equidae. In some instances the progressive evolution of the stage 

 is traced .through gradual transition ; in others it appears more abruptly. 

 The older genera in several instances continue along with the newer 

 stages, paralleling them, but less progressive or divergent in their adapta- 

 tion. The emphasis must be placed, therefore, on the first appearance of 

 a genus, not on its extinction. A single upper molar or premolar of a 

 Tertiary equid can usually be positively identified as to genus ; the lower 

 molars are someAvhat less certainly determinable. 



Various other groups of mammals corroborate this series of life zones 

 so fully and exactly that the evidence is beyond question. Their genera 

 are not always so exactly limited in range; they are not always so cer- 

 tainly recognizable from fragmentary material. And in none is so long 

 and continuous a sequence of evolutionary stages traceable. But the suc- 

 cession of Tertiary life zones in the continental formations of North 

 America may be considered as firmly and permanently fixed. 



The correlation of these zones with the marine Tertiaries, and in par- 

 ticular with the European standard of nomenclature, is somewhat less 

 positive and precise. It must be recognized, in the first place, that the 

 European standard is not as definitely and precisely fixed as one could 

 wish. European geologists are not in accord as to exactly where tlie lines 

 should be drawn in the marine succession between the different Tertiary 

 epochs ; they are still less in agreement as to the exact correlation between 

 the marine and the terrestrial faunas of western Europe, and the succes- 

 sion of the terrestrial faunas of Europe is not so clearly demonstrated by 

 stratigraphic evidence as it is in this country. Were these points defi- 

 nitely cleared up, it would be practicable to make very precise final corre- 

 lations between the European and American Tertiaries. As it is, there is 

 a certain amount of libration according as we accept one or another Euro- 

 pean authority for the standard. While I believe that our American 

 nomenclature must needs conform to the European standards, so far as 

 those standards are fixed and universally agreed on, yet I think that the 

 more exact zonal division might well be based on American standards, 

 where, as in this instance, the succession is more exactly and certainly 

 shown than in Europe. 



The correlation with the marine succession in this country is not so 

 satisfactory. In the East we have a small mammalian fauna of Oligocene 



