SHELLS AND MOLLUSCOUS ANIMALS. 215 



sands of birds, and found as many thousands of 

 living shell-fish scattered on the surface of a heap 

 of shells, that for aught I know, would have filled 

 as many thousand wagons." 



The shell of our common limpet is sub-oval, of 

 a yellowish or greenish ash-colour, and marked 

 with many ribs. Should the reader .happen to 

 find on the frond of the Oar- weed, or any other of 

 our large olive sea-weeds, a beautiful clear little 

 shell, shaped something like that of the common 

 limpet, of an olive-green colour, with blue inter- 

 rupted rays, he may recognise it as the Pellucid 

 limpet (Patella pellucida). 



Then there are the shell-fish, commonly called 

 the Key-hole limpets, of the genus Fissurella, some 

 of them frequent, and all easily known by the 

 aperture at the summit of the shell, shaped exactly 

 like a key-hole. This aperture is exactly over 

 the breathing apparatus, and serves to conduct the 

 water necessary for respiration. 



On the rock on which we find the limpet, we shall 

 probably also see some one of the several species 

 of the canoe shell, as they abound on rocks between 

 the high and low- water marks, often being com- 

 pletely embedded in them. They also attach them- 

 selves to the stems of the coarser sea-weeds, and 

 to the hulls of ships. These Chitons have a very 

 remarkable shell, formed of eight distinct portions, 

 arranged in a row down the back, and so similar 

 to the plates of ancient armour, that this animal 

 is often called coat-of-mail. When we take it up 

 in the hand, it rolls up in a ball like the common 

 wood-louse. It has very sharp little teeth in its 

 mouth, with which to nibble sea-weeds or other 

 food. The species are difficult to determine, as 



