626 MOLLUSCA -CLASS II.-PELECYPODA. 



left valve. The siphona are short and deeply fringed ; the foot very long 

 and slender. 



The cockles, Oaediid.'e, live buried in sand or sandy mud, and in some 

 species with a prickly shell the size and number of the spines is dependent 

 on the nature of the sea bottom, the spines being fewest in those which 

 bury in the firmest silt. The shells of the genus Hemwardium are the most 

 peculiar in shape of almost any bivalve, for they are much shorter than wide 

 or thick. • 



The "giant clam" (Tridacna), and the bear's-paw {Hippopus), so commonly 

 used to make shell-baskets, form the family TridacjiiDjB. In these no 

 anterior adductor is present in the adult, nor are there any siphons. The 

 shell is ornamented with radiating ribs, and the margins are deeply 

 indented. The hinge has a single cardinal in each valve, and two posterior 

 laterals in the right, one in the left valve. Tridacna is the largest bivalve 

 known, either recent or fossil, T. gigas attaining 2 ft. in length. It lives 

 in coral reefs moored by its huge byssus, which issues through the large 

 notch in the valves in front of the umbones. The animal is most gaudily 

 coloured, rivalling the corals amongst which it dwells. Large dull pearls are 

 sometimes found in it. Hippopus, on the other hand, has a close-fitting 

 shell, through it spins a small byssus. It, too, is a reef-dweller. 



The ChamidyE, which resemble oysters in external appearance, and in 

 being attached, are inequivalve, with sub-spiral umbones and external 

 ligament. The shell is rough, and generally ornamented with frills, or flat 

 shelly projections. Sometimes one valve is attached, sometimes the other ; 

 the hinge teeth consist of two cardinals in the attached, and one in the free 

 valve, with traces of laterals. They inhabit tropical seas. 



Sub-order 5. — Mtacea. The shells of this group gape more or less widely, 

 and never completely enclose the animal. The mantle margins are united, 

 except where passage is given for the foot, and at the siphons, which are long 

 and more or less united. The hinge is degenerate. The typical family, 

 MyidcE, includes the common Mya or "gaper" of our coasts. The animal 

 is difficult to obtain owing to its habit of burrowing a foot down in the mud. 

 Its siphons are encased in an extension of the horny periostracum, and are 

 never completely withdrawn into the shell. The hinge is peculiar : from the 

 left valve at the umbo a spoon-shaped process projects that carries the 

 resilium, the corresponding attachment in the right valve being underneath 

 the umbo, the hinge plate being folded quite back. Traces of the cardinal 

 teeth are sometimes visible. 



In the SoLENiD^, or razor-shells, the shell is usually very elongate and 

 open at either end. The ligament is external, the hinge teeth are small, 

 usually two or three in each valve. The foot is long and cylindrical ; the siphons 

 short, and mostly united. The family is world-wide in its distribution. The 

 common Solen of the English shores is known to all. The razor-fish live 

 deeply bedded in sand or mud, into which they force themselves by means 

 of their powerful foot. By some the Solenidoe are placed as a separate 

 group nearer the Tellinidae. , 



The SAXicAViDiE resemble the Myidfe, but the gills extend into the cavity 

 of the inhalent siphon. The genus Glycimeris also resembles Mya in its 

 habitat ; but the members of the typical genus Saxicava bore into hard 

 stone, and are generally found in colonies, honeycombing the rocks with 

 burrows sometimes 6 in. in depth, nor do they scruple to drill through 



