Heart-Cockle and Fresh-water Cockle 1 1 1 



accustomed to a certain admixture of fresh water, 

 and have gradually wandered farther up the rivers 

 and their feeders until they have finally become used 

 to water that is absolutely fresh. As they have got 

 into quieter waters, where, too, enemies are less 

 abundant, their shells have become thinner and less 

 solid, with modifications of the animal to match. 



The Pearl Mussels (Unio) are represented in our 

 waters by three species, of which the best known is 

 the Painter's Mussel (U. j^^cioriim), so called be- 

 cause the valves were formerly used to hold artist's 

 colours, just as they are used to-day to hold the gold 

 and silver for illum- 

 inating. The oblong 

 compressed shell is 

 covered by a thin 

 epidermis through 

 which show the yel- 

 low ground-tint, the 

 parallel lines of 

 brown, and the rays of green on the hinder part. 

 The beaks are considerably forward from the centre ; 

 the lower margin is almost straight. The interior 

 is pearly, and the muscular impressions are distinct. 

 A notched tooth on the front of the left valve fits 

 into a groove formed by the margin of the shell and 

 a strong tooth on the right valve ; whilst a long 

 plate-like tooth on the back of the right valve fits 

 into the groove of a similar plate on the left valve. 

 The animal has a large, broad, tongue-shaped foot of 

 a reddish or yellow colour, with wliich it burrows into 

 the mud of ponds and slow rivers. ' Owing to the 

 thinness of the epidermis on the beaks — these being 



Painter's Mussel 



