208 PALEONTOLOGY. 



influenced the grouping and distribution of extinct faunas 

 and floras in former periods of its history. 



The preservation of the generic and specific characters ot 

 fossil remains varies in different strata ; hence arise many 

 of the difficulties which beset the investigations of the palae- 

 ontologist, and warn him against a too common error of draw- 

 ing hasty generalisations from incomplete and imperfect data. 



Many fossil remains are preserved as complete and perfect 

 for scientific investigation as the bodies of living animals. 

 For example, the elephant and rhinoceros, with their muscles, 

 skin, and hair preserved by the antiseptic power of cold in 

 the frozen drift of Russia and Siberia ; or the skeletons of 

 chelonian and saurian reptiles interred in the oolitic, creta- 

 ceous, and tertiary rocks. 



Most of the fossil shells of the tertiary period, with the 

 exception of their colours, have their specific characters as 

 perfectly preserved as those of our present seas. In many 

 of the secondary and paleozoic rocks, they are likewise 

 found in great perfection ; whilst in other strata the exist- 

 ence of these organisms is determined by casts of the inner 

 surface of the shell, or by moulds made in the solid rock ; 

 the calcareous material of the shells having been subse- 

 quently removed by chemical action. 



The horny and calcareous skeletons of articulated animals 

 are sometimes preserved entire ; as insecta in amber, and 

 Crustacea in the palaeozoic, secondary, and tertiary rocks. 



The complicated skeletons of radiated animals are found 

 interred in strata of different ages ; and the minute details 

 of the structure of this group are so perfectly preserved, 

 that the fossil species of Crinoid, Asteroid, and Echinoid 

 radiata, are described with as much rigorous accuracy as the 

 existing species of these orders. 



The numerous remains of delicate polypiferous zoophytes, 

 which are strewed among rocks of all ages, are preserved 

 with wonderful perfection, and indicate, in some instances, 

 the former existence of coral reefs in the seas of the silurian 

 and oolitic periods ; whilst the microscopic organisms that 

 have left their siliceous shields in the tertiary beds attest, to 

 the observer of Nature's minute works, how permanent are 

 the specific characters of the world that lies beyond the ken 

 of the unaided eye of mm 



