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 af 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 Cardiidce, 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 (b). 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 



VALVE AND SOFT-PARTS OF Tridacna. 



