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 or Fosstz SHeiis.—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, withany 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 
