Kinds of Fossils in Determining Geological Age. 485 



If we take the Ammonites as another example from the 

 invertebrates, the case is totally different. From the earliest 

 appearance of this family, the members have been constantly 

 changing, developing new genera and species, each admirably 

 adapted to mark definite zones or horizons, and already used 

 extensively for that purpose. 



The Trilobites offer another example of a group of inverte- 

 brates ever subject to modification, from the earliest known 

 forms in the Cambrian, to the last survivors in the Permian. 

 They, too, are thus especially fitted to aid the geologist, as 

 each has distinctive features, and an abiding place of its own 

 in geological time. 



The above examples are all marine forms, and from their 

 abundance, wide distribution, both in time and space, are 

 among the best of all witnesses in marking the succession and 

 duration of changes in geological history. 



If we turn now to the fresh-water Mollusca, we find among 

 them little evidence of change from the Palaeozoic forms to 

 those still living, and can therefore expect little assistance from 

 them in noting the succeeding periods during their life-history. 



Among the fossil Yertebrates, the same law as to specializa- 

 tion holds good. The value of particular groups as witnesses 

 of geological changes depends largely on their own suscepti- 

 bility to change, and this is equally true of single genera and 

 species. There are indeed some primitive vertebrates, espe- 

 cially among the Fishes, that appear to have changed little 

 during their geological life. The genus Lepidosteus is a good 

 illustration, and hence it is of limited value as evidence of 

 what has taken place during its known geological history. 

 Other fishes, however, are much better witnesses of the past. 



The Reptiles as a class offer still better evidence of geologi- 

 cal changes, and in many instances may be used to advantage 

 in marking horizons. The great sub-class of Dinosaurs, from 

 their beginning in the Triassic, show marked changes of devel- 

 opment throughout the whole of Mesozoic time. During the 

 Cretaceous, highly specialized forms made their appearance, 

 and at the close of this period when all became extinct the last 

 survivors were the strangest of all, reminding one, in their 

 bizarre forms, of the last 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 Pterodactyles, and many 

 other extinct reptiles, each according to the degree of special- 

 ization attained. 



