HALF AN HOUR WITH SHELL-FISH (BIVALVES). 223 



yellow rays. In shape it is nearly round, having 

 the beak on one side. There are several British 

 species of this genus. Very characteristic of the 

 northern seas are the Astartes, of which we have 

 several. One of them, called sulcata, is exceedingly 

 variable in colour and form. The surface of the 

 valve is alternately ridged and furrowed ; hence the 

 specific name. In compressa, another species, the 

 name again is a good indication of the chief feature 

 about it, as the shells are flat or compressed. The 

 latter are only about half an inch in length. The 

 hinge of all the Astartes is very strong and pecu- 

 liarly formed, occupying more space than is usual in 

 shells of so small a size. Living under pretty nearly 

 the same circumstances, only resembling the Arte- 

 mis in shape and general appearance, is the Lucina, 

 of which one species, lorealis, reaches its northern 

 limit in our seas. It is an elegant little shell, 

 ornamented with fine concentric lines. Allied to a 

 few of the foregoing, as regards its northern character, 

 but much larger in size, is the Cyprina Islandica 

 Indeed, this is one of our largest and stoutest shells, 

 well proportioned, and having very stout and strong 

 hinge-teeth. Not unfrequently it attains a length 

 of four to five inches. It is most abundant on our 

 northern coasts, and off those of North Wales. 

 When recently thrown ashore, it is covered with a 

 dark olive-brown membrane. This is another of the 

 species peculiar to some of our boulder clays, or 

 " glacial " deposits. 



