38 HALF-HOURS IN THE GREEN LANES. 



beak, the valves being kept together by a ligament. 

 This will always enable the student to identify it 

 from Unio, in the hinge of which there are distinct 

 and well marked articulations. The " Swan mussel/ 

 (Amjdon cygneus) is the largest of all our fresh-water 

 shells, often attaining the length of four or five 

 inches. The heron you may have disturbed on your 

 approach to the tarn, had most probably been making 

 a hearty meal off the swan-mussels, for both it and 

 the common crow are very partial to them. Pennant 

 records that they will carry the closed shell to a 

 height and drop it, in order to break it open in 

 this ingenious fashion ! There are several species 

 of Unios in British ponds, of which, perhaps, the 

 commonest is the " painters' mussel," ( Unio pie- 

 toruni), so called because its valves were formerly 

 used by the Dutch painters for holding their colours. 

 The pearl mussel (U. margarUiferus) is fonder of 

 running water, and is generally found in rivers in 

 the neighbourhood of mountain ranges. In the same 

 runnel leading to the tarn that you found Bithinia 

 you will doubtless find both Cyclas and Pisidium, 

 small bivalve molluscs, with nearly globular shells. 

 The latter genus is noted for its single siphon. In 

 the stream, just where the bend has drifted fine sand 

 into a heap, you may surely reckon on meeting 

 with myriads of their empty shells, which have been 

 swept here by the current. 



Whilst collecting or noticing the habits of these 

 humble creatures, one cannot but remember the 



