192 FEESH-WATEE AQUAEIA. 



of the water inhabited by these molluscs as well as on their 

 supply of food." He arranged the five species in the following 

 order : 



1. Pisidiuin amnicum i 



2. P. fontinale \ ^^^^ triangular sheUs. 



3. P. pusillum has an oval shell. 



4. P. nitidum has a round shell. 



5. P. roseum has an oblong shell. 



The bodies of aU the species of Pisidium are very much 

 alike; their colour is whitish or whitish-grey. 



Pisidium amnicum (Fig. 127), by far the largest of the 

 genus, is found in the slow-running and stagnant water of 



nearly every part of Britain. 

 The shell is glossy, solid, of 

 a greyish horn-colour, and is 

 a little over Jin. in length. 

 This mollusc is said to spend 

 the greater part of its time 

 Fig. 127. Pisidium amnicum. partly buried in the mud at 



the bottom of the water. 

 Pisidium, fontinale is much smaller than the last species, 

 being only about |-in. in length. The umbones of the shell 

 are prominent. This mollusc lives in the slow-running and 

 stagnant water of nearly every part of Britain, and is described 

 as being the most active species of the genus to which it 

 belongs. It is said to be fond of small splashy ponds ; hence, 

 I suppose, its specific name. 



Pisidium pusillum lives in weedy water throughout Britain. 

 The shell is more oval in shape than that of the last species, 

 and even tinier in size ; hence the specific name. 



Pisidium nitidum has an almost round shell, the umbones 

 of which are marked with rather deep grooves. It is found 

 in clear, stagnant water in many parts of Britain, but it 

 is not so common as some of the other species. Its funnel- 

 shaped siphon is generally considered as a help to distinguish 

 it from its other relatives, all of which have such a confusing 

 family likeness. The sheU is iridescent and very glossy ; hence 

 the specific name. 



