CONCHIFERA. 241 



form of oysters, scallops, mussels, and cockles.* They come next to the 

 univalves (gasteropoda) in variety and importance, and though less numerous 

 specifically, are far more abundant individually.! The bivalves are all 

 aquatic, and excepting a few widely-dispersed and prolific genera, are all 

 inhabitants of the sea ; they are found on every coast, and in every climate, 

 ranging from low -water mark to a depth of more than 200 fathoms. 



In their native element the Oyster and Scallop lie on one side, and the 

 lower valve is deeper and more capacious than the upper ; in these the foot 

 is wanting, or else small, and not used for locomotion. Most other bivalves 

 live in an erect position, resting on the edges of their shells, which are of 

 equal size. Those which move about much, like the river-mussel, maintain 

 themselves nearly horizontally^ and their keel-shaped foot is adapted for 

 ploughing through sand or mud. The position of those bivalves which live 

 half-buried in river-beds or at the bottom of the sea, is often indicated by the 

 darker colour of the part exposed ; or by deposits of tufa, or the growth of 

 sea-weed on the projecting ends of the valves. 



In Nucula and some others the foot is deeply cleft, and capable of ex- 

 panding into a disk, like that on which the snails glide : whilst in the mussel, 

 pearl-oyster, and others which habitually spin a byssus, the foot is finger-like 

 and grooved. 



The burrowing species have a strong and stout foot with which they bore 

 vertically into the sea- bed, often to a depth far exceeding the length of their 

 valves ; these never voluntarily quit their abodes, and often become buried 

 and fossilized in them. They most usually burrow in soft ground, but also 

 in coarse gravel, and firm sands and clays ; one small modiola makes its 

 hole in the cellulose tunic of Ascidians, and another in floating blubber. 



The boring shell-fish have been distinguished from the mere burro wers, 

 perhaps without sufficient reason, for they are found in substances of every 

 degree of hardness, from soft mud to compact limestone, and the method 

 employed is probably the same. 



The means by which bivalves perforate stone and timber has been the 

 subject of much inquiry, both on account of its physiological interest, and 

 the desire to obtain some remedy for the injuries done to ships and piers and 

 breakwaters. The ship-worm (teredo) and some allied genera, perforate 

 timber only ; whilst the pJiolas bores into a variety of materials, such as 



* They are the Dithyra of Aristotle and Swainson, and constitute the second or 

 sub-typical group in the quinary system. 



t It has been stated that the predatory mollu&ca are more numerous than the 

 vegetable-feeders; but it is not so with the individuals constituting the species. 



I This is the position in which they are always figured in English books, being 

 best suited for the comparison of one shell with another. 



See the admirable memoir by Mr, Albany Hancock, in the An. Nat. Hist, for 

 October, 1848. 



