MOLLUSCA, THE SHELL-FISH 107 



that the snapping to of the lid is capable of pro- 

 ducing a sound, which may perhaps be audible at 

 a distance under the water. Various molluscs 

 have been credited with producing sounds, either 

 by muscular movements or by the grating of the 

 shell as the animal walks. The common Tortoise- 

 shell Snail, Helix asflersa, sometimes makes a most 

 alarming noise when crawling over a window. It 

 has been disputed whether the sounds thus made 

 are produced by the grating of the creature's 

 tongue-ribbon on the glass, as it files off small 

 particles of algae and vegetable moulds, which 

 are invisible to our eyes : or whether they are 

 sounds due to suction of the muscular surfaces, 

 such as may be produced by drawing a wet finger 

 across glass. The noise, however produced, is, 

 as I can testify from experience, sufficiently loud 

 and weird to be very startling, if heard in the dead 

 of night. 



Turn now to the Bivalves or Lamellibranchiate 

 molluscs, which include the familiar oyster, cockle, 

 and mussel. These are also known as the Pele- 

 cypoda, and as the Aglossa, or molluscs without a 

 tongue-ribbon. The name Lamellibranchiate re- 

 fers to the shape of the gills " plate-like," or 

 flat ; the name Pelecypoda to the shape of the 

 foot, " hatchet-foot." 



The animal usually chosen as a type of these, 

 the fresh-water mussel, is rather a dull sort of 

 creature, so we have chosen a prettier and more 

 lively specimen as a representative of the class; 

 namely, one of the Scallops, Pecten opercularis, 

 sometimes called the Quin, the shell of which is 

 shown in the frontispiece of the book. This is 

 one of the most beautiful, perhaps the most beau- 

 tiful, of the English shells. The generic name, 



