1 62 T.AMEI.LIBRANCHS. 



friends who live far from the coast may feel dis- 

 couraged at this statement, but many of them can try 

 the same experiment with a fresh- water mussel, 

 which they can get from some pond or stream. In 

 bivalve molhisks which live above the mud the foot is 

 small or absent. 



All ordinary bivalve mollusks take their name 

 from the lamellar or plait-like form of the four gills 

 or branchicE^ hence they are called Laniellibraiichs^ 

 La-mel'-li-branks. The young are hatched from 

 minute eggs, and usually spend their earliest days 

 within the mantle of the parent. They are free 

 swimming little creatures, and when breathed out 

 into the surrounding water they sport around for a 

 little time, become separated, and then settle down to 

 dig a burrow, or attach themselves to some object 

 above the mud. Their shells soon begin to grow, and 

 they quickly take up the humdrum life of their 

 ancestors. 



The shell which I began to describe has a deep 

 pallial sinus, as shown in the engraving, and this fact 

 indicates that it lives a good way below the surface of 

 the sand or mud, and that it has long siphons. The 

 shells are rather flat and thin, and are marked exter- 

 nally by many fine lines radiating from the unibo, 

 'and these are crossed by small concentric ridges, 

 which correspond to lines of growth. The cardinal 

 hinge-teeth are near the anterior extremity of the 

 shell, which is always opposite the pallial sinus; and 

 the ligament is long and external. 



The length of a pair of bivalve shells is the dis- 

 tance from one end to the other, parallel to the hinge 

 line. The height is the distance from the hinge line 

 to the opposite edge of the shell; and the breadth is 



