568 Notices of Memoirs — Prof. 0. C. Marsh — Age of Fossils. 



stages of the Ammonites, their cotemporaries. The Crocodiles, too, 

 show great changes during Mesozoic time, and are thus of much 

 ■value in determining geological horizons. So, also, are the Ptero- 

 dactyles and many other extinct reptiles, each according to the 

 degree of specialization attained. 



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 character's 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 mammals from the Cretaceous; all are 

 of special value as witnesses of the past. 



During Tertiary time, however, the enormous development of the 

 class of mammals, their rapid vai'iations, and, most important of all, 

 the highly specialized characters they develop, offer b}'^ far the best 

 evidence of even the smaller changes of climate and environment 

 that mark their life-history throughout. 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 hoi"se tribe. Higher 

 up in the Eocene Orohippiis occurs, and still higher comes Epih'ppiis, 

 near the top of the Eocene. Again, through the Miocene more 

 genera of horses, Mesohippus, Miohippus, and others, follow in 

 succession ; and the line still continues in the Pliocene, when the 

 modern genus Equus makes it appearance. 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 tliat every 

 experienced palaeontologist 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 detei-mining the 

 relative 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. 



