THE BIVALVES 



3385 



smooth, in the beautiful Cardissa they are flattened, heartshaped, and keeled at the 

 sides. The true clams ( Tridacnidtz) 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 (oQ, branchial (a), 

 and anal (). 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 terminating 

 on the edge of the valves in pointed projections. 

 The genus Tridacna contains the largest of all 

 bivalves, T. gigas sometimes measuring more 

 than a yard in length, and weighing as much as 

 five hundred pounds. The animals are gor- 

 geously colored, and a mass of them nearly a mile 

 in extent has been compared to a bed of tulips. 

 The six or seven species are found in hot lati- 

 tudes, such as the Red Sea, and Indian and Pacific 

 Oceans. The adductor muscle is said to be good 

 eating. Hippopus differs from Tridacna in hav- 

 ing no gape in the anterior end of the shell for 

 the passage of a byssus. H. maculatus is one of VAI.VE AND SOFT PARTS OF Tridacna. 

 the most common shells used as ornaments. The 



Chamidce are remarkable for their strong irregular oyster-like shells, which are 

 often brilliantly colored, and covered with spines or ridges like the thorny oysters. 

 The shells exhibit two well-marked muscular scars, strong hinge teeth, and an ex- 

 ternal ligament. These bivalves inhabit tropical or subtropical seas, and are 

 usually attached by one of the valves to rocks. The animal has the margins of the 

 mantle united, excepting at the siphonal openings and the pedal orifice. To a 

 mollusk leading a stationary life, and not given to spinning a byssus, a foot would 

 appear to be useless; nevertheless Chama possesses a reduced form of this member, 

 but what purpose it serves it is difficult to conjecture. Some of the fossil members 

 of this family, Diceras and Requienia, for example, have remarkable shells, quite un- 

 like those of the existing forms. 



SUBORDER Myacea 



In the family Psammobiidtz the typical genus Psammobia has the siphons very 

 long, slender, and separated as in Tellina, the foot large and tongue-like, and the 

 edges of the mantle fringed. The shells are long and narrow, compressed, slightly 

 gaping at both ends, generally somewhat obliquely truncate posteriorly, often 

 brilliantly colored, and beautifully sculptured. Four species occur on the British 

 coasts. The gapers {Myidce) take their title from their widely gaping shells, which 

 are covered with a wrinkled periostracum extending also over the siphons; these 

 being united their whole length, and fringed at the ends. My a arenaria, a common 

 British species, also abounds on the sandy shores and mud flats of the Eastern 

 States of North America, where it is eaten in quantities. The clams, as they are 



