236 MOLLUSCA. 



ginning of the present century, directed their attention to 

 the condition and distribution of the fossil species. Nearly 

 six hundred species of recent shells have been described as 

 natives of Britain, while the fossil species furnished by the 

 strata of the different formations, and which have been ac- 

 curately described, fall greatly short of that number. There 

 is, however, reason to believe that the fossil species are even 

 more numerous than the recent ones. 



It would have been a pleasant task for us to have entered 

 into the details of this most important subject, but our limits 

 permit us only to trace its outlines. Our remarks, however, 

 we trust, will prove useful to those who are entering this 

 fruitful field of investigation, and will embrace some obser- 

 vations on the systematic characters, condition, situation, 

 and distribution of these organic remains. 



SYSTEMATICAL HISTORY OF FOSSIL SHELLS The de- 

 termination of the characters of fossil shells is attended with 

 no inconsiderable amount of difficulty. The changes which 

 they have undergone, and their union, in many cases, with 

 the substance of the rock, having become incorporated with 

 it, prevent us from ascertaining, with any degree of accuracy, 

 the peculiar marks by which the species can be character- 

 ised. No trace of the animal remains to aid us in the inves- 

 tigation, so that all our distinctions must depend upon the 

 characters furnished by the shell. This circumstance should 

 prevent us from placing much confidence on the conclusions 

 which have been drawn with respect to the resemblance be- 

 tween fossil species, and those which still exist in a living 

 state. 



The difficulty of determining the fossil species, and the 

 reluctance to form new genera, rendered the descriptions 



