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CONCHIFERA? 393 
CHAPTER IV. 
CLASS VY. CONCHIFERA, Lamarck. 
(Lamelli-branchiata, Blainyille.) 
The bivalve shell-fish, or Conchifera, 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 indiyidually.t ‘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 le 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 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, hke 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 haye 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 
inthem. They most usually burrow in soft ground, but also 
incoarse 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-typical group in the quinary system. 
+ It has been stated that the predatory modlusca are more numerous than the 
veretable-feeders ; but it is not so with the individuals constituting the species. 
t This is the position in which they are always figured in English books, being best 
suited for the comparison of one shell with another, 
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