HALF AN HOUR WITH SHELL-FISH (BIVALVES). 221 



been alluded to (Fig. 121). This shell may frequently 

 be found at low water, buried near the lowest ebb, 

 in the sands in an upright position, with its skinny 

 siphon or tube protruding. Empty shells are also 

 to be picked up on the beach, and may be recognised 

 by the large hinge, one shell having an enormous 

 toothed projection, and the other a corresponding 

 depression. They are frequently called "Gaper- 

 shells," because, when denuded of the external skin, 

 the shells will not meet at one end. In the " Trun- 

 cated " species (Mya truncata), one side of the shell 

 is much abbreviated or truncated, and the siphon 

 much longer (Fig. 122). This species has a northern 

 distribution, reaching its southern limits on our 

 coasts. Thracia is a genus of bivalves having 

 some relation to the foregoing, and possessing a 

 large hinge, with a peculiar structure. One shell is 

 humpy, and the other nearly flat ; both are very 

 fragile. The " Otter-shells " (Lutraria) are the 

 largest of British bivalves, some of them averaging 

 as much as five inches in length. They resemble 

 the " Razor-shells " in some respects, principally in 

 their inability to close the ends of their shells. 

 They are, however, more nearly allied to Mactra. 

 All the " Otter-shells " live in the mud, almost com- 

 pletely imbedded there. There are several species 

 living in our seas, the commonest of which is 

 elliptiea. 



Tapes is a genus of bivalves, beautifully coloured, 

 and which is further adorned with fine lines that 



