• CONCHIFEUA. 393 



CHAPTEE IV. 



CLASS Y. COKCHIFEEA, Lamauck. 



{LamelU- hrancMafa^ Blainville . ) 



The bivalve shell-fish, or Concliifera, are familiar to every 

 one, under the 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. f 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, of 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, X 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 seaweed 

 on the projecting ends of the valves. 



In Nucula and some others the foot is deeply cleft, and 

 capable of expanding into a disk, like that on which the snails 

 glide ; whilst in the mussel, pearl-oyster, and others which 

 habitually spin a hyssus, 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 volun- 

 tarily quit their abodes, and often become buried and fossilised 

 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. 



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

 sub-typiual group in the quinary system. 



t It has been stated that the predatory mollusca are more numerous than the 

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



X This is the position in which they are always figured in English books, being bes6 

 suited for the comparison of one shell with another. 



s3 



