TRANSACTIONS OF SECTION C. 871 



The Trilobites offer another example of a group of invertebrates ever subject 

 ^0 modification, from the earliest known forms in the Cambrian to the last sur- 

 vivors in the Permian. They 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, and wide 

 ■distribution, both in time and space, are among the best of all witnesses in marking 

 the succession and duration of changes in geological hi.story. 



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

 ■evidence of change from the earliest 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 A'ertebrates the same law as to specialisation 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 Lepi- 

 ■flosteus 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 

 inuch better witnesses of the past. 



The Reptiles as a class offer still better evidence of geological changes, and 

 in many instauces may be used to advantage in marking horizons. The great 

 sub-class of Dinosaurs, from its beginning in the Triassic, shows marked changes 

 ■of development throughout the whole of Mesozoic time. During the Cretaceous 

 highly specialised forms made their appearance, and at the close of this period 

 when all became extinct the last survivors were the strangest of all, reminding 

 ■us, in their bizarre forms, of the last stages of the Ammonites, their contem- 

 poraries. The Crocodiles, too, show great changes during Mesozoic time, and are 

 tlius of much value in determining geological horizons. So, also, are the Ptero- 

 dactyles and many other extinct reptiles, each according to the degree of specialisa- 

 tion attained. 



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

 geological time, as their changes and the high degree of their specialisation 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 

 .give only 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 value as witnesses cf the past. 



During Tertiary time, however, the enormous development of the class of 

 mammals, their rapid changes, and, most important of all, the highly specialised 

 ■characters they develop, oft'er by far the best evidence of even the smaller changes 

 ■of climate and environment that mark their life history throughout. The Ungu- 

 lates alone will answer the present purpose as an illustration, and even one group, 

 Ihe horses, will make clear the point I wish to emphasise. 



Near the base of the Eocene the genus Eohipptcs is found, representing the 

 •oldest known members of the horse tribe. Higher up in the Eocene Orohippus 

 •occurs, and still higher is Epihippus, near the top of the Eocene. Again through 

 the Miocene more genera of horses, Mesokipjnts, Miohippus, and others, follow 

 jn succession, and the line still continues in the Pliocene, when the modern genus 

 Equus makes its 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 that 

 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 



