spire-shells and Marsh-snails 237 



inner side. When gliding through the waters the 

 branchial plume is exserted from beneath the mantle, 

 and with it a long tentacle-like outgrowth from the 

 mantle, which is regarded as also employed in 

 respiration, and thought by some as the other 

 branchial plume aborted. The sexes are united in 

 each individual, but only one sex is functionally 

 active at one time. They inhabit slow and still 

 fresh waters all over these islands. The 

 Common Valve-shell (F. piscinalis) has a 

 blunt spire, and the general form of the 

 shell is globular, brownish yellow in colour, 

 and marked with spiral ridges. The um- 

 bilicus, though round and deep, is not large. The 

 Flat Valve-shell (F. cristata) differs in the form of 

 its greyish shell, which is always flat, like that of a 

 Planorhis, and the animal has a smaller snout and 

 stouter tentacles. The umbilicus, too, is proportion- 

 ately larger and more open. 



