SEA-SHELLS ON THE SEASHORE 143 



"clams," and, again, the snails, periwinkles, whelks, 

 and limpets. It is the shells of these animals which 

 are " true " shells in the sense in which the word is used 

 by " collectors " of shells, and in the' sense in which we 

 speak of " the shells of the seashore." These shells are 

 usually very hard, solid things, made up of layers of 

 lime-salts and horny matter mixed, and they remain for 

 a long time undestroyed, washed about by the currents 

 of the sea, and thrown up on to the beach, after the soft, 

 oozy creature which formed them — chemically secreted 

 them on its soft skin — has decomposed and disappeared. 

 They are readily distinguished into two sorts — (i) those 

 which are formed in pairs, or " bivalves," each member 

 of the pair being called a "valve"; and (2) those which 

 are single, or " univalves," often spirally twisted, as are 

 those of snails and whelks, but sometimes cap-like or 

 basin-like, as are the shells of the limpets. There is not 

 so great a difference between bivalve and univalve shells 

 as there seems to be at first sight. For if you examine 

 the pair of shells of a mussel or a clam when they are 

 quite fresh, you will find that the valves are joined 

 together by a horny, elastic substance, and are, in fact, 

 only one horny shell, or covering, which is made hard 

 by lime deposited on the right and on the left, as two 

 plates or valves, but is left soft and uncalcified along a 

 line where these two valves meet, so as to allow them 

 to move and gape, as it were, on an elastic hinge. It is 

 the fact that the two valves of the shell of the bivalve, 

 lying right and left on its body, correspond to the single 

 shell of the snail or limpet, which differs from the bivalve- 

 shell in not being divided along the back by a soft part 

 into right and left pieces. That there is this real agree- 

 ment between bivalve and univalve molluscs is quite 

 evident when we examine the soft animal which forms the 

 shell and is protected by it. 



