M. B. de Basterot's Observations on Fossil Shells. 105 



in a neighbouring basin. He will ask himself whether these 

 species are distinct or are mere varieties, and he will find very 

 few means of answering this question. 



The difficulty here offered is of a very special nature, and 

 deserves particular attention. This difficulty consists in the 

 variation in form observable among shells, both living and 

 fossil, when we examine the same species obtained from dif- 

 ferent situations. 



We never, it may be affirmed, observe perfectly identical 

 species, either on the surface of the earth (at least in the more 

 recent beds) or on the coasts of our present seas, in situations 

 separated by considerable horizontal distances, or even simply 

 distinct from a difference in the nature of the rock or waters ; 

 a little greater or a little less elongation throughout all its 

 parts, prominence in the striae and the tubercles, size in the 

 folds, &c. are always met with, and render the determination 

 of species a very difficult task. 



In well characterized species, the habit of exact observation 

 is necessary to seize these differences ; but among a number 

 of other species these differences are so remarkable, that dif- 

 ferent names have often been given to the same shell, from 

 characters which were in fact merely accidental*. 



There must however be limits to these local variations ; and 

 I am far from being of the opinion of those, who consider that 

 all species may merge into each other : at the same time I am 

 much inclined to believe that the limits between the species 

 and the variety, so difficult to assign in the higher classes of 

 animals, is still less easily fixed among shells. This, in fact, 

 should occur, for " the differences constituting the variety de- 

 pend on determined circumstances, and their extent augments 

 with the intensity of these circumstances f." Now shells, 

 more than any other class, are the slaves of circumstances ; 

 they cannot withdraw themselves from the action of exterior 

 influences. If a change in food and habitation can alter, as 



• This may be easily observed in many genera: the following is an in- 

 stance taken from the Pleurotomce. All the assumed following species are 

 probably only modifications of a single real species, produced by the cir- 

 cumstances under wliich the animals inhabiting these shells were placed. 



Pleurotoma oblonga Brocchi, pi. viii. fig. 5. . Lives in the Mediterranean. 



ejusd. var. ...Id pi. ix. fig. 19. . Fossil intheDuchyofRtggio. 



acumiftata,..Sov/eTby,[)\. 387,fig. 2. Foss. Muddyford.. England. 



- brcvirostris Id pi. 1 46, fig. 4. Foss. Highgate. 



■ mullinudu ...Lamarck Foss. Grignon. 



- rjiisd. var... Nob Foss. Knvirons of Bordeaux. 



AfonHt7«. ...Defrance . . M.S. . . . Foss. Fleury la Riviere, near 



Epernay. 

 ■f Cuvicr, Reck, stir let Oss. Foss., due. pri'lim., 5J(I edit. p. [)8. 



iSexu Series. Vol. 2. No. 8. Aug. 1827. P they 



