414 
MOLLUSCS. 
sinus. In Circe, on the contrary, the mantle-impression is unsinuated. The 
shells of Tapes are longer than the typical forms; the foot is long, grooved, 
and frequently furnished with a byssus, the siphons being separate at the 
ends and beautifully fringed. They are most abundant in the warm seas of 
the Eastern Hemisphere, but four species range as far north as England. The 
large species of the allied genus Cyrena are found in brackish water at the 
mouths of rivers, and in mangrove-swamps; while the smaller forms known 
as Corbicula, Sphcerium, and Pisidium occur in fresh water in most parts of 
the world. The shells of the last two groups are very similar, but the animals 
are readily distinguished by the number of the siphons. In Sphcerium there are 
two which are united at the base, but separated at the extremities, whereas in 
Pisidium only a single anal siphon is present. In this genus the water is 
conveyed to the gills through the pedal opening of the mantle. There are four 
British species of Sphcerium, and five of Pisidium, all of which possess the 
faculty of floating in an inverted position at the surface of the water, or they 
suspend themselves from the surface by a fine byssal thread. One species (P. 
pusillum) does not require a constant, or even a frequent supply of water, often 
living at the roots of bog-moss and grass, and it has also been found between the 
bark and wood of fallen trees in moist places. 
Suborder Cardiacea. 
The Carcliidce, Tridacnidce, and Chamidce are the principal families belonging 
to this suborder of which there are living representatives, but a few extinct 
families, such as the Hippuritidce, are also considered to belong here. The cockles 
(Cardiidce ) abound in shallow water in most 
parts of the world, where there are sheltered 
sandy bays. Some of the exotic forms are 
beautifully sculptured, and their colours also 
are often very bright and varied. The animals 
have short fringed siphons, and the long foot is 
bent and used for leaping. Probably many are 
eatable, like the common cockle. In addition 
to this kind, nine species are found on the 
British coast. The typical species of Cardium 
have the convex valves ribbed, the ribs inter¬ 
locking at the margins. In Lcevicardium the 
shells are smooth, in the beautiful Cardissa they 
are flattened, heart-shaped, and keeled at the 
sides. The true clams ( Tridacnidce ) differ from 
other bivalves with united mantle - margins in 
having only a single adductor muscle, like the 
oyster, the anterior being obsolete. The mantle 
has three distant openings, pedal (d), branchial (a), and anal (6). The foot is 
small, finger-like, and capable of producing a stout byssus ( e ). The shells are 
equivalve, ponderous, with a few stout ribs radiating from the umbones, and 
