British species of Cijclas and Pisidiam. 291 



large number of specimens, collected from different sources, in 

 order to determine the characters of a single species with any 

 degree of precision. Occasionally the shell becomes exceedingly 

 ventricose at the expense of its height, which is thereby consider- 

 ably diminished; and the valves which perhaps naturally meet 

 at an acute angle, under such circumstances meet at an obtuse 

 one. This is particularly the case with one or two varieties here- 

 after to be described. Neither can sculpture be relied upon, the 

 strias varying exceedingly in number and distinctness according 

 to the nature of the water in which the shell is found: a circum- 

 stance of which Dr Leach was not sufficiently aware when he 

 formed three species out of Pisidimn amnicum. Age likewise 

 produces great changes: not only are young shells much more 

 compressed than adult ones, but in many instances the relative 

 proportions of their parts are different. Indeed in the case of 

 the minute species, so great and general a similarity prevails 

 amongst their young, that it is hardly possible to identify them 

 in this state without the closest examination. 



After what has been stated, I need hardly add, how cautious 

 I have been in characterizing the minuter species of this group, 

 and that it is not till after repeated observations upon very ex- 

 tensive series of each, including many varieties from different 

 localities, that I venture to bring forward the following list of 

 such as are found in this country, as one which I trust will 

 be found more complete than any which has appeared hither- 

 to. 



It will be observed, that with respect to the arrangement of 

 these shells, I have deviated from that of Lamarck and most 

 authors in referring them to two genera. This I have done in 

 conformity with the views of Pfeiffer, who in his excellent work 



oo2 



