486 O. C. Marsh— Different Kinds of Fossils, etc. 



The Mammals, however, are by far the most important class 

 for marking geological time, as their changes and the high 

 degree of their specialization furnish the particular characters 

 that are most useful to the geologist in distinguishing definite 

 zones, and the more limited divisions of the strata containing 

 their remains. The few mammals known from the Trias are 

 so peculiar that they can only give us hints of what mammalian 

 life then was, but in the Jurassic the many forms now known 

 offer important testimony as to the different horizons in which 

 their remains are found. This is true also of the known mam- 

 mals from the Cretaceous ; all are of special value as witnesses 

 of the past. 



During Tertiary time, however, the enormous development 

 of the class of m mammals, their rapid variations, and, most 

 important of all, the highly specialized characters they develop, 

 offer by far the best evidence of even the smaller changes of 

 climate and environment that mark their life-history through- 

 out. The ungulates alone will answer the present purpose as 

 an illustration, and even one group, the horses, will make clear 

 the point I wish to bring before you. 



Near the base of the Eocene the genus Eohippus is found, 

 representing the oldest known member of the horse tribe. 

 Higher up in the Eocene Orohippus occurs, and still higher 

 comes Epihippus, near the top of the Eocene. Again through 

 the Miocene more genera of horses, Mesohippus, Miokippus, 

 and others, follow in succession, and the line still continues in 

 the Pliocene, when the modern genus Equus makes its appear- 

 ance. Throughout this entire series, definite horizons may be 

 marked by the genera, and even by the species of these equine 

 mammals, as there is a change from one stage to the other, 

 both in the teeth and feet, so that every experienced paleon- 

 tologist can distinguish even fragments of these remains, and 

 thus identify the zones in which they occur. 



This is true of every group of mammals, although not to 

 an equal extent, so that in this class we have beyond question 

 the best means of identifying the age of Tertiary strata by 

 their fossil remains. 



I have thus briefly pointed out some of the evidence on 

 which a decision may be reached as to the value of the different 

 kinds of fossils, Plants, Invertebrates, and Vertebrates, in deter- 

 mining the age of strata, All evidence of this kind is of 

 value, but it is the comparative value of each group that is the 

 important point I wish to emphasize, and I have brought the 

 matter before this Section of the Association in the hope that 

 a better understanding on this question may be reached among 

 geologists, in the interest of the science to which we are all 

 devoted. 



