"SHELL-FISH." 187 



can, by a little thought, get some notion of their habits by 

 examining the empty shells that are cast up within the littoral 

 zone by heavy seas. Those that are fresh and clean externally, 

 though without any signs of wear from long washing among 

 the shingle, may be safely regarded as burrowers that habitu- 

 ally lie beneath the sand or mud. These, too, will be found 

 to have both valves of the shell almost, if not quite, equal in 

 size and shape^ whilst those which, like the Oyster and the 

 Scallop, lie upon the sea-bottom, have very unequal valves, 

 the under one being deeper and concave, whilst the upper 

 valve is flat and more brightly coloured, to harmonise with its 

 surroundings. Often, too, this exposed upper valve will be 

 crusted with acorn-shells, Serpulce, Sertularice, or seaweeds. 



It may prevent confusion further on if we now say a few 

 words by way of denning the parts of a bivalve shell, its latitude 

 and longitude, and its relation to the animal whose vital activi- 

 ties produced the valves. The Spiny Cockle, or Red-nose 

 (Cardium aculeatuni) of our" illustration, is on its back. If we 

 were to take it, or any other bivalve-shell, and turn it the 

 other way, so that the hinge connecting the two- valves was 

 uppermost, we should have it in the natural position. 



A bivalve mollusk is an inferior creature to a limpet or a 

 winkle, because these have heads with eyes, but the bivalve has 

 not. In the larval condition it has eyes, but by a retrograde 

 movement like that of the cirripedes, it gets rid of these as 

 useless in the life it is to live henceforth. But in spite of its 

 want of a head, we know which is its anterior and its posterior 

 end, its dorsal and its ventral surface ; and with our know- 

 ledge of the relation of animal and shell, we are not troubled 

 to open the valve to look at the creature, when we wish to 

 describe the parts of a shell. It will be noticed that each 

 valve curls over near the hinge and takes a form not greatly 

 unlike a beak. This is more strongly marked in some species 

 than in others; anyhow, it is popularly known as the beak, 

 though it is technically distinguished as the umbone, or boss. 



