CARBONICOLA. 37 
§ III. CARBONICOLA. 
It is always a difficult matter to decide as to which characters in a group of 
shells are to be regarded as of definite generic or specific value. Classifications, 
though based on anatomical resemblances, are, as Herbert Spencer states, only 
“subjective conceptions which have no absolute demarcations in nature 
corresponding to them ;” so that it comes to be almost a purely personal matter to 
decide on what forms shall be considered for utilitarian purposes as of specific 
rank, or what others shall only be estimated as varieties. 
I have felt that it was necessary as an aid to determine the horizons of such 
important beds as those of the Coal-measures to give specific rank to any forms 
which seemed to be typical of a bed; in other cases, when in the same beds a 
series of varieties occurred, to include them under one species. 
The great difficulty in the subject hes in the very variable shape and characters 
of shells from the same beds ; and were it not for the fact that I have fortunately 
been able in nearly every case to study hundreds of specimens, I probably should 
have been tempted to double the number of named forms. 
A larger amount of variation might have been expected to have occurred in 
forms of this group obtained from the different coal-fields, but as a matter of fact 
the local variation is far greater. Amongst fresh-water shells there cannot exist 
the means for that free intermixture and consequent maintenance of a more 
constant type which obtains with marine mollusca. They would of necessity be 
isolated by drainage systems; and it is difficult to see, in the absence of aérial 
animals, how any amount of dispersion could take place except by floods. It 
is difficult to account for excessive variation in gregarious organisms with an 
identical environment ; and this tendency to vary would appear to be universal, 
from the large numbers of forms that have been described and named by previous 
’ 
authors. 
The beds in which the bivalve mollusca of the Coal-measures occur are 
indurated marls, black shales, and ironstones ; and in most cases the fossils have 
both valves preserved in a closed position, showing that they are in the place and 
position in which they lived. They generally he with their long axes parallel to 
the lines of stratification ; but there exist beds in nearly every coal-field composed 
of the crushed valves and débris of myriads of shells which appear to have been 
finally deposited after death of the animal, the valves being nearly all single, or 
if double, widely opened out flat. Asa rule, the lower beds of the Middle Coal- 
measures (stage F of Prof. Hull) are the most prolific in molluscan remains. I 
can find no evidence of any shells, except a compressed Anthracomya (A. Phillipsii) 
