4 INTRODUCTION. 



position, and mode of occurrence of the rocks from which Palae- 

 ontology obtains its materials. Lastly, Palaeontology, apart from 

 its own importance as an independent branch of Zoology, is em- 

 ployed by the geologist to assist him in his determination of the 

 chronological succession of the materials which compose the crust 

 of the earth. Palaeontology, therefore, in one of its aspects, is a 

 branch of geological science, and requires separate study in its 

 relation to historical Geology. 



Definition of Fossils. 



All the natural objects which come to be studied by the palaeon- 

 tologist are termed " fossils " (Lat. fossus, dug up). In most cases, 

 fossils, or, as they are often termed, " petrifactions," are actual por- 

 tions of animal or vegetable organisms, such as the shells of Molluscs, 

 the skeletons of Corals, the bones of Vertebrate animals, the wood, 

 bark, or leaves of plants, &c. ; and these may be preserved very 

 much in their original condition, or may have been very much 

 altered by changes subsequent to their burial. Strictly speaking, 

 however, by the term " fossil " is understood " any body, or the 

 traces of the existence of any body, whether animal or vegetable, which 

 has been buried in the earth by natural causes " (Lyell). We shall 

 find, therefore, that we must include under the head of fossils ob- 

 jects which at no time themselves formed parts of any animal or 

 vegetable, but which, nevertheless, point to the former existence of 

 such organisms, and enable us to reason as to their nature. Under 

 this head come such fossils as the moulds or " casts " of shells and 

 the footprints or markings left by various animals upon sand or mud. 



In the great majority of cases fossils are the remains of animals or 

 plants which are now extinct — that is to say, which no longer are in 

 existence, but have entirely disappeared from the earth's surface. 

 In some cases, however, fossils are the remains of recent animals — 

 that is, of animals which are still found in a living condition upon 

 the globe. The term " sub-fossil," sometimes applied to these, has 

 been more appropriately applied in another sense, and is best dis- 

 carded in this connection. In any case, the fact that a given speci- 

 men belongs to an extinct species of animal or plant, or that it is 

 referable to some existing form, does not enter in any way whatever 

 into the determination of the question as to whether or not it is 

 truly a fossil. If such a specimen is found in those portions of the 

 earth's crust which we can show by other evidence to have been 

 formed prior to the establishment of the existing terrestrial order, 

 then it is a fossil ; while any remains, even though belonging to the 

 same animal, which are found in deposits which have been formed 

 during the historical period, would be, strictly speaking, referred to 



