PART II. CHAPTER XIII. 



167 



Classification of the Tertiary Formations. 



by imperceptible gradations into other forms, considered by many 

 zoologists and botanists as entitled to rank as distinct species. — 

 The difficulty of defining the limits in such cases is not greater, 

 perhaps, in conchology than in other departments ; but it happens 

 that this science has advanced very rapidly since the year 1830, 

 when M. Deshayes drew up the tables published in the Principles 

 of Geology. In that year he had it in his power to refer to no 

 more than 5000 species of recent shells then in Paris ; but the 

 number of species now in the public and private collections of 

 Europe has increased to between 8000 and 9000; and, what is 

 of no less consequence, individuals of species which before that 

 time were extremely rare, have been supplied in abundance. 

 Fossil shells also have been collected with equal zeal and suc- 

 cess ; and thus the facility of discriminating nice distinctions in 

 closely allied species, or of deciding which characters are con- 

 stant and which variable, has been greatly promoted ; and the 

 study of these more ample data has led all conchologists to 

 separate many species, both fossil and recent shells, which before 

 they had confounded together. 



In consequence of the changes of opinion brought about by 

 these additions to our knowledge, it has become necessary not 

 only to examine all the newly discovered fossil and recent tes- 

 tacea, but also to reconsider all the species previously known. 

 As this laborious task has not yet been executed by M. Deshayes, 

 engaged as he is in other scientific labours, I am unable at present 

 to offer to the reader the improved results which the revision of 

 the tables drawn up in 1830 would afford. In the mean time I 

 have obtained the aid of several eminent conchologists, and in 

 particular of Dr. Beck, of Copenhagen, in comparing a great 

 number of the recent and fossil shells which had been identified. 

 By this investigation I have come to the conclusion that the per- 

 centage of recent species in a fossil state is decidedly less, espe- 

 cially in the older tertiary strata, than was indicated in the list 

 published in 1833. A large number, in particular, of the forty- 

 two species of Eocene testacea, to which the names of recent 

 shells were given in the tables, cannot be considered as identical, 

 if we adopt the same standard of specific distinctions as is recog- 

 nized in the new edition of Lamarck's conchology, edited by M. 

 Deshayes himself, in 1836. 



But although many corrections are indispensable, and the 

 proportion of recent species found fossil in the Eocene, Miocene, 

 and older Pliocene strata may be considerably less than was at 

 first supposed, we have no reason on this account to feel dis- 

 couraged in an attempt to found the classification and nomen- 

 clature of the tertiary periods on the great principle before 



