216 HALF AN HOUR WITH SHELL-FISH (BIVALVES). 



also, is very rough and rasp-like. The shell is 

 composed of that form of carbonate of lime termed 

 arragonite, which is much harder and more endur- 

 able than the ordinary kind. The foot being 

 attached, the animal keeps up a constant swaying 

 to and fro, in a semicircular fashion, bringing the 

 rasp-like ends of the valves to act mechanically on 

 the rock. Meantime the siphon is busy squirting 

 forth the water containing the fine mud thus abraded 

 away. This is the simple means by which the 

 PTiolas manages to make its tunnel, as witnessed 

 several times in the aquarium. Mr. Eobertson, 

 who exhibited Pholades in the act of boring into 

 the chalk, has described the means as " A living 

 combination of three instruments, viz., hydraulic 

 apparatus, a rasp, and a syringe !" There is yet 

 another kind of borer, much smaller in size, which 

 literally honeycombs the seaward faces of lime- 

 stone rocks, and causes no small denudation of them, 

 by burrowing. This is the Saxicava, of which there 

 are several species. The shell is very brittle, and 

 covered with sharp wrinkles or ridges. The common 

 name given to the PTiolas by fishermen is " Piddock." 

 Several species are found in the rocks of the British 

 coast. 



Strewn almost everywhere on thousands, in the 

 spaces between high and low water, are certain 

 shells, generally devoid of strong markings, smooth, 

 and having only delicate shades of pink colouring 

 to recommend them. These are the Madras, or 



