16 RAMBLES IN SEAECH OF SHELLS. 



on moss-covered stones and amongst dead leaves and 

 debris in woods. 



Having so far stretched our model to its utmost 

 while reducing it in size and width, let us now re- 

 store it to the shape in which we found it, that is, to 

 the shape of a common Helix, and attempt some 

 modifications in another direction. Instead of elon- 

 gating let us compress the shell vertically, flattening 

 the spire and all the whorls until the shape is that 

 of an ammonite, or, to nse a more familiar simile, a 

 catharine-wheel. We have then a rough notion of 

 what the genus Planorbis (i.e., flat coil) is like. 

 In this genus there are said to he eleven British 

 species, although it requires a nice discrimination to 

 identify what are, except in regard to size, but eleven 

 slight modifications of the common type of which we 

 have endeavoured to give some outline. 



The largest of them is Planorbis corneus, being 

 in diameter, when adult, about the size of a shilling, 

 and in thickness about a quarter of an inch. The 

 others are all very much smaller and flatter, varying 

 in depth from the thickness of a penny to that of 

 a knife-blade. This genus has some remarkable 

 peculiarities, which have been well pointed out by 



