Heart-Cockle and Fresh-water Cockle 95 



mud — so little that tliey need no long siphons ; 



sufficiently, however, to make the presence of ribs 



upon the shell an advantage — these helping to keep 



them firmly in place when they have withdrawn the 



foot and closed the valves. 



In the Iceland Cyprina {CyiJvina islandica) we 



have an example of a single species constituting the 



sole living representative 



of its genus, though of 



the fossil species of that 



genus we know nearly a 



hundred. The shape of 



the valves is shown in the 



figure, but if the complete 



shell be looked at from 



behind it will be seen that 



— in that aspect — it is 



heart - shaped. It is a 



thick and slightly glossy shell, with numerous 



irregular concentric raised lines, which are stronger 



towards the margins than in the middle. The colour 



is white tinged with yellow and pink, but the brown 



epidermis hides this. The beaks are prominent and 

 considerably carved. Along the 

 upper edge of each valve there 

 is a groove which holds one side 

 of the ligament. The hinge-plate 

 is broad and bears three strono- 

 cardinal teeth. The laterals are 

 also strong in the left valve, but 

 in the right they are very slight. 



It measures between 4 and 5 inches in length, 



and almost as much from the beak to the lower 

 7 



Iceland Cyprina 



Interior of Cyprina 



