50 M. DESHAYES' TABLES OF SHELLS. [Ch. V. 



tions, and having been witness to the great time and labour 

 devoted by him to this arduous work, I feel confidence in the 

 results, so far as the data given in his list will carry us. It was 

 necessary to compare nearly forty thousand specimens, in order 

 to construct these tables, since not only the varieties of every 

 species required examination, but the different individuals, also, 

 belonging to each which had been found fossil in various loca- 

 lities. The correctness of the localities themselves was ascer- 

 tained with scrupulous exactness, together with the relative 

 position of the strata ; and if any doubts existed on these ques- 

 tions, the specimens were discarded as of no geological value. A 

 large proportion of the shells were procured, by M. Deshayes 

 himself, from the Paris basin, many were contributed by dif- 

 ferent French geologists, and some were collected by myself 

 from different parts of Europe. 



It would have been impossible to give lists of more than 

 three thousand fossil shells in a work not devoted exclusively 

 to conchology ; but we were desirous of presenting the reader 

 with a catalogue of those fossils which M. Deshayes has been 

 able to identify with Jiving species, as also of those which are 

 common to two distinct tertiary eras. By this means a com- 

 parison may be made of the testacea of each geological epoch, 

 with the actual state of the organic creation, and, at the same 

 time, the relations of different tertiary deposits to each other 

 exhibited. The number of shells mentioned by name in the 

 tables, in order to convey this information, is seven hundred 

 and eighty-two, of which four hundred and twenty-six have 

 been found both living and fossil, and three hundred and fifty- 

 six fossil only, but in the deposits of more than one era. An 

 exception, however, to the strictness of this rule, has been made 

 in regard to the fossil shells common to the London and Paris 

 basins, fifty-one of which have been enumerated by name, 

 though these formations do not belong to different eras. 



It has been more usual for geologists to give tables of cha- 

 racteristic shells ; that is to say, of those found in the strata 

 of one period and not common to any other. These typical 

 species are certainly of the first importance, and some of them 



