INTRODUCTION. 



Of all the genera of British non-marine Mollusca none has 

 presented more difficulties to the student than Pisidium. The 

 small size of the shells, their great variability, the lack in most 

 cases of any striking external characteristics, as well as the con- 

 fusion in which the subject has been left by the various authorities, 

 have all contributed to these difficulties, with the result that the 

 genus has been largely neglected. 



Very early in our joint work on the post-tertiary Mollusca, 

 Mr. A. S. Kennard and I were led to see that the group must bo 

 dealt with, whilst the late Dr. 0. Bottger had urged me to study 

 it more closely. It was not, however, till Dr. A. C. Johansen 

 visited this country in 1901 that any serious start was made. 

 Dr. Johansen it was who directed my attention to the hinge- 

 characters as the only sure means of identification of the species, 

 and by his intimate knowledge of the Scandinavian forms he was 

 able to point out that we had species present in Britain hitherto 

 unrecognized. 



The investigation of the British species of Pisidium was to have 

 been our joint task, but unfortunately for Malacology, Dr. Johansen 

 on his return to Copenhagen was drafted to other zoological 

 work, and this attempt to unravel a very tangled skein was left 

 to me. 



