CARBONICOLA. 37 



§111. 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 lies 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 aerial 

 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 lie 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 debris 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. As a 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 Avthracomya (A. Phillipsii) 



