3384 THE MOLLUSKS OR SHELLFISH 



SUBORDER Veneracea 



In the Veneridce, which form the first family of this group, the animal has 

 rather short, more or less united siphons, with fringed openings. The mantle is 

 open in front for the passage of a tongue-like foot, which is sometimes furnished 

 with a byssus. The shells are solid, equivalve, and often beautifully colored and 

 sculptured, with the hinge teeth large and divergent, and the ligament external. 

 In the typical genus Venus the shells are more or less circular, globose, and often 

 have the surface cancellated, and the inner edge of the valves crenulated. There 

 are several British forms of Veneridce, but none are eaten to any great extent. On 

 the contrary, the large Venus mercanaria the clam of the Atlantic States of 

 North America is sold in large quantities in the fish markets of New York and 

 Philadelphia. In Dosinia the shells are more compressed, circular, and marked 

 within with a deep, narrow, pallial 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 Cor- 

 bicula, Sphcerium, and Pisidium occur in freshwater in most parts of the world. The 

 shells of the last two groups are very similar, but the animals are readily distin- 

 guished 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 

 Sph(zrium, 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 colors 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 eat- 

 able, 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 interlocking at the margins. In Lcevicardium the shells are 



