Notices of Memoirs — Prof. 0. C. Marsh — Age of Fossils. 567 



on the spot where they grew, with their most characteristic parts 

 preserved, may furnish important evidence as to their own nature 

 and geological age. Characteristic examples are found among the 

 plants of the Coal-measures, in the Cycads of Mesozoic strata, and in 

 the fossil forests of Tertiary' and more recent deposits. 



The value of all fossils as evidence of geological age depends 

 mainly upon their degree of specialization. In the Invertebrates, for 

 instance, a Linguloid shell from the Cambrian has reached a definite 

 point of development from some earlier ancestor. One from the 

 Silurian or the Devonian, or even later formations, however, shows 

 little advance. Even the recent forms of the same group have no 

 distinctive characters sufficiently important to mark geological 

 horizons. 



If we take the Ammonites as another example from the inverte- 

 brates, 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 invertebrates 

 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 MoUusca, we find among 

 them little evidence of change from the Palseozoio 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 Vertebrates the same law as to specialization 

 holds good. The value of particular groups as witnesses of 

 geological changes depends largely on their own susceptibility to 

 change, and this is equally true of single genera and species. 

 There are, indeed, some primitive vertebrates, especially 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, how'ever, are much better 

 witnesses of the past. 



The Eeptiles as a class offer still better evidence of geological 

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

 marking horizons. The great subclass of Dinosaurs, from their 

 beginning in the Triassic, show marked changes of development 

 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 



