THE VENUS FAMILY. 245 



FAMILY XIIL CYCLADID^l. 



The animal has a thin, horny shell, with the mantle lobes partly open for the passage of a 

 large protruded foot, and united posteriorly to form the branchial and excurrent siphons, which are 

 prolonged into tubes wholly or partially united. 



In Cyclas the animal is ovo- viviparous. The gills are large, the valves of shell are nearly equal, 

 and much inflated. The young of Cyclas are hatched in the gills of the parent. They vaiy in size 

 from one-sixth to one-quarter the length of the mother. They are very active, climbing in aquatic 

 plants, attaching themselves by delicate threads. They chiefly inhabit the temperate regions of the 

 globe in both hemispheres. 



The shells of Cyrena are covered with a rough epidermis ; they are oval and thick, and have 

 three hinge teeth, and one lateral tooth in each valve ; the foot is strong and tongue-shaped. This 

 mollusc is found abundantly in the Nile and other Eastern rivers to China, and in mangrove swamps, 

 usually near the coast. It is particularly interesting to geologists, being found in the old river 

 deposits of the Thames, &c., associated with the remains of elephants, &c. 



FAMILY XIV. ASTARTID^E. 



The characters in this family are, shell free ; oblong or nearly round ; surface of valves often 

 concentrically ribbed, and covered with a brown epidermis ; hinge with strongly developed cardinal 

 teeth. All the genera are marine. 



The shell of Astarte is thick ; the valves are somewhat round, and compressed towards the "beaks ; 

 they are smooth or concentrically furrowed ; there are two hinge teeth in each valve ; the hinge ligament 

 is external. Of the twenty species known, by far the larger number are Arctic, being met with by the 

 Polar expeditions, both living and as dead shells, on raised beaches far above the present level of the sea. 



Genus Crassatella. The shell is oblong, attenuated behind ; the valves are very thick, smooth, or 

 furrowed concentrically ; the ligament is internal ; the muscular impressions are deep, rounded, distinct ; 

 the mantle line is simple. Thirty-four species are living in Australia, the Philippines, Africa, &c. 



FAMILY XV.- CYPRINIDJE. 



About half of this family are fossil, and the rest were more abundant in the Tertiary period 

 than at the present time. The valves are equal, round, or elongated, solid, closely-fitting, with a 

 thick dark epidermis ; the hinge ligament is external ; there are hinge teeth, one and three in each 

 valve ; the mantle border is simple. 



In Cyprina the shell is large and strong, oval in outline. Like Astarte, Cyprina has an 

 extreme northern range from Britain to Iceland, and northward as far as the explorers have 

 advanced towards the Pole. 



The gemis Circe has a thick, compressed oi'bicular shell, ornamented with diverging striae ; the 

 valves are compressed, and the umbones flat ; hinge teeth three and three. Forty living species 

 are found in Australia, India, the Red Sea, Britain, &c. 



In the " Heart Cockle " (Isocardia) the valves are smooth, inflated ; the umbones are distant, 

 and somewhat spiral ; the hinge ligament is external ; hinge teeth two and two, lateral teeth one and one, 

 in each valve. It burrows in the sand. Five species are living in the Mediterranean, China, and Japan. 



Cyjrricardla dwells in the crevices of rocks and coral, and is found in the Red Sea and Indian Ocean. 



In Cardita the hinge ligament is external, the margins are toothed, the hinge teeth are one and 

 two ; the shell is narrow and oblong, and radially libbed. Fifty-four species are found living, chiefly 

 in tropical seas. It had a more extensive distribution in past geological times. 



In Subdivision 2 the animals have long respiratory siphons, and the pallia! border is recurved. 



FAMILY XVI. VENERID.E. 



These bivalves are free, and do not live attached to other bodies by a byssus, nor do they bore 

 into rocks, but live simply in sandy or muddy sea-bottoms, into which they burrow by means of their 

 tongue-shaped foot. The siphons are unequal retractile ; the hinge ligament is external ; the shell 

 is regular and closely fitting, more or less circular or oval in outline, with three teeth in each valve. 

 The shells of this family are hard, solid, and generally marked by elegance of form and coloration. 



In Venus the mantle margins are fringed ; the siphons are unequal and separate ; the shell is thick 



