BIVAL VES. 
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terminating on tlie 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 500 lbs. The animals are gorgeously 
coloured, 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 latitudes, such as the Red 
Sea, and Indian and Pacific Oceans. The adductor muscle is said to be good 
eating. Hippopus differs from Tridacna in having no gape in the anterior end of 
the shell for the passage of a byssus. H. maculatus is one of the most common 
shells used as ornaments. The Ckamidce are remarkable for their strong irregular 
oyster-like shells, which are often brilliantly coloured, and covered with spines or 
ridges like the thorny-oysters. The shells exhibit two well-marked muscular scars, 
strong hinge-teeth, and an external 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 mollusc leading a stationary life, and not given to 
spinning a byssus, a foot would appear to be useless; nevertheless Cliama 
possesses a reduced form of this member, but what jmrpose it serves it is difficult 
to conjecture. Some of the fossil members of this family, Diceras and Bequienia. 
for example, have remarkable shells, quite unlike those of the existing forms. 
Suborder Myacea. 
In the family Psammobiidce 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 coloured, and beautifully sculptured. Four species occur on the British 
coasts. The gapers (Myidee) 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. Mya 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 commonly 
called, live in deep burrows in the sand or mud, the shells often being a foot below 
the surface. A recent writer observes that when the flats are covered with water, 
the clams extend their long siphons up through the burrow to the surface of the 
sand, and through one of these tubes the water and its myriads of animalcules are 
drawn down into the shell, furnishing the gills with oxygen, and the mouth with 
food, and then the water, charged with carbonic acid and faecal refuse, is forced 
out of the other siphon. Two species of Mya constitute the staple food of the 
walrus. The Solenidce, or razor-shells, are also great sand-burrowers, and indeed 
bore with such rapidity, and to such a depth, that they often elude capture. They 
possess very elongate shells, and are remarkable for the great development of the 
foot. They not only burrow in sand, but also have the power of darting through 
the water like scallops. They are eaten by the poorer coast population. In the 
Saxicavidce the species of the typical genus Scixicava are some of the few bivalves 
which have the power of boring into limestone and other soft rocks, although they 
