148 INTRODUCTION TO ZOOLOGY CHAP. 



This genus is common in rivers and ponds. 

 Unio margaritifer is only found in rapid streams in the 

 mountainous districts of the North of England, and in Scot- 

 land, Wales, and Ireland. It is this species which may 

 produce valuable pearls. 



S haerium Orb-shell cockles are small forms with almost 



or " Orb- ' equilateral shells (i.e. with the umbo almost in 



shell the centre of the dorsal margin), and with distinct 



Cockles." ninge _ tee th, botn "cardinal" and "lateral." The 



valves of the shell are so convex as to give the name " orb- 



shell " to this genus. 



The margin of the mantle, round both the exhalent and 

 inhalent apertures, is drawn out into a short siphon (see 

 Fig. 93). 



Sphaerium corneum (the Horny Orb- 

 shell) is very common in ponds and 

 ditches. A full-grown specimen may be 

 J an inch long. 



Sphaerium lacustre is also very common 



FIG. ^.-Sphaerium in England. It is rather smaller than 

 corneum. ,111 j 



the last, and more compressed. 



Pisidium, or I n this genus the shell is smaller than in 



"Pea-shell Sphaerium, and the umbo is no longer in the 



Cockle." cen t, re of the dorsal margin, but behind it. There 



is only one siphon, the exhalent siphon. The shell has hinge- 



teeth, as in Sphaerium. 



There are several fairly common species, none with a shell 

 more than J of an inch in length, and the differences between 

 them are not very marked. 



Marine Bivalve Molluscs (Oysters, Scallops, Cockles, etc.). 



Oysters ^he Common Edible Oyster (Ostrea edulis) is 



of economic importance, because of its popularity 

 as an article of food. 



Oysters, when adult, are incapable of locomotion. The 

 little free-swimming larvae, which are liberated from the 

 parent as "oyster spat," quickly disperse, and after a time 

 each settles down on some hard surface, lying on its left 

 valve, which quickly becomes cemented down, so preventing 

 further movement. This valve becomes thick and deeply 



