SPKCIKS OF PISIDIUM. O 



reflects the animal than does the Gastropod, and the muscle-scars 

 in some cases, though not, seemingly, in Pisidwm, and especially 

 the hinge furnish reliable data. 



Bourguignat was, we believe, the first to insist on the utility of 

 the hinge for broadly classifying the species of Pisidium (21, p. 82), 

 but unfortunately he did not realise the applicability of its detailed 

 characteristics for the purpose of the determination of species. 



Jeffreys, who was acquainted with Bourguignat's paper, over- 

 looked, or did not give sufficient heed to, the pregnant suggestion, 

 and it was Clessin (35) who first, in theory at all events, made 

 ti?e of the detailed hinge -characters for the determination of 

 species. 



Under the circumstances, when entering upon the researches of 

 which the results are set forth in the following pages, it seemed 

 advisable to ignore in the first instance all names whatsoever, to 

 ascertain simply how many distinct forms of the genus could be 

 differentiated, taking the details of the hinge -structure as a 

 guide, and to leave the question of their final nomenclature till the 

 last. 



Many thousands of specimens, recent and fossi', were studied ; 

 many hundreds of the recent forms had to be opened by boiling in 

 a test-tube in weak caustic soda, before they could be examined 

 under the microscope (1-in. objective), and a very large number of 

 the fossil forms had to be scrutinized three or four times, individually, 

 under a lens. 



In the end seventeen well-marked forms were discriminated, of 

 which two are extinct, whilst one has not as yet been found, either 

 recent or fossil, in these Isles, although it possibly does occur, 

 thus leaving fourteen species known to be living in Britain. 



To attempt subdivision of the genus and allot sectional names 

 on so few forms is to court disaster more complete than has 

 overtaken others with more extensive material to hand. 



Clessin (35) divided the genus into three groups : 



1. FLUMJNINA, type : P. a-tnnicum, Mull. 



2. KIVULINA, type : P. sujriinnn, Schmidt. 



3. FOSSA IUXA, type : P. obtusale, C. Pfr. 



His first group, as Sterki (166) has already shown, was based 

 on a misapprehension of the cardinal tooth of the light valve of 

 P. amnicam, which he thought was double ; whilst his other groups 

 prove to be incongruous mixtures. 



Westerlund (184) followed Clessin, but added a totally un- 

 necessary fourth group for the " deep-water" forms. 



Ball's grouping (49) unhappily docs not give promise of any 

 greater permanence, and one can only hope .that Dr. Sterki, who is 

 working at the American species, may, with the plentiful examples 

 at his command, be able to attain better results. 



The seventeen forms in question in this paper are capable, 

 however, of being roughly classed. Thus P. antnicum and 



