THE COCKLE FAMILY. 243 



from tlie mud an abundant stream of bubbles of sulphuretted hydrogen. In such situations the 

 spiral shells for example, Bithynia have lost the ends of their spires, and the cliscoidal shells, 

 like Planorbis, have been found with a small hole caused by the dissolution of the inner whorls. 

 The great and ponderous Mussels of the American rivers, and even the fresh-water Unios and 

 Anodons of British stream?, are often externally eroded, and the cause has been the subject of much 

 speculation. The umbo is the part first formed, and consequently that where the epidermis is 

 thinnest and has been longest exposed to the action of the elements, and it is this portion of the shell 

 which is most corroded. 



Division b. SIPHOKIDA. In this section the animals have respiratory siphons, and the lobes of 

 the mantle are usually united. In the first subdivision the siphons are short and the pallia! border is 



simple. 



FAMILY IX. CHAMID.E. 



The shells in this family are thick, the valves unequal, the hinge teeth two in one valve and one in 

 the other ; the ligament is external. 



Genus Chama. These shells are found only in tropical seas among coral reefs. They are 

 attached indifferently by either valve. When the right valve is fixed the dentition is reversed, 

 the left valve having the single tooth. The exterior of the valves is ornamented with a succession 



of brightly-coloured frills. 



FAMILY X. TEIDACNID^E. 



The valves are strongly ribbed and toothed at the margin, the hinge ligament is external, the 

 shells are equal. Sometimes the animal is attached by a byssus, in others it is free. 



The genus Tridacna is the largest of the whole class of bivalves. The Giant Clam 

 (Tridacna gigas) of 

 the Indian Ocean, 

 the shell of which 

 often weighs upwards 

 of 500 Ibs., contains 

 an animal weighing 

 sometimes 20 Ibs., 

 which is stated by 

 Captain Cook to be 

 very good eating. 



Darwin, in his " Voy- THIDACNA SQUAMOSA A, orxsiDE, AXD B, INSIDE OF SHELL. 



age of a Naturalist 



Round the World," in describing Keeling Atoll says " We stayed a long time in the lagoon, 

 examining fields of coral and the gigantic Clam-shells, into which if a man were to put his 

 hand he would not as long as the animal lived be able to withdraw it." The Paphian Venus, 

 springing from the sea, is usually represented as issuing from the opening valves of a Tridacna. The 

 huge valves of this shell are frequently used for holy water in churches. Two weighing 500 Ibs. and 

 measuring more than two feet across may be seen in the church of S. Sulpice, Paris. (Dillwyn.) 



FAMILY XI. CARDIAD.E. 



The Cockles live unattached ; their valves are equal and nearly bilaterally symmetrical ; the 

 surface of the shells is ribbed radially ; the siphons are short ; there are two gills on each side , 

 the foot is large and recurved. 



In Cardium the shell is inflated, the umbones prominent ; the hinge has two lateral teeth, one 

 in each valve. The margins of the valves are crenulated. The common Cockle (Cardium edule) is 

 largely used in many parts of England for ^food. It is obtained at extreme low water on all sandy 

 shores, living buried in the sand. It ranges from the Baltic southward, and is found in the Black 

 Sea and Caspian. Two other much larger species occur on the British coast, viz., C. rusticum, and 

 the Prickly Cockle (C. aculeatum). Both of these are edible, but the small species is the one so largely 

 consumed in all parts of Britain. 



FAMILY XIL LUCTNIDJE. 



The valves of the shell of this family are circular, closely fitting, and unattached ; the surface 

 of the shell is dull ; the foot is long and cylindrical. 



