BIVALVES. 415 



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 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 Chamidaz 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 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 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 (Myidw) 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 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 Solenidw, 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 

 Saxicavidxz the species of the typical genus Saxicava are some of the few bivalves 

 which have the power of boring into limestone and other soft rocks, although they 



