39 



though the general form of the shells is more 

 simple, the depositions of the mantle are as lux- 

 uriant in spires, rays, ribs, and frills, as in 

 the Gasteropoda, as may be seen in the Spnndylus, 

 the Pectens, clams, and many others, and perhaps above all- 

 the Hammer Oyster. The two shells of the bi-valves are 

 commonly called upper and lower, but this definition only 

 holds good with such shells as are fixed like the oysters. 

 The shells of those which are free are called by naturalists 

 right and left. When the bi-valves are at rest they lie on 

 the left valve, and in one species— the Pecten Pleuronectes 

 — the left valve is without colour, like the under side of the 

 flat fishes. To know which is the right, and which is the 

 left valve of a shell, place it on its lower edge, with the 

 Umbo forward, and you have the two shells in their proper 

 position. When the molluscs betake themselves to loco- 

 motion, they rise on their edges, and move forward on 

 a foot, or what serves as a foot. The common Cockle can 

 do more than this, for by means of a fold in its foot, it can 

 leap with some agility ; and the Pectens, swim after a 

 fashion by clapping their shells together. The hinge of the 

 bivalve shells is not a hinge in the mechanical sense, it is 

 only a strong ligament, which opens the shell by its elas- 

 ticity, and the muscular action of the animal is exerted, not 

 to open the shell, but to close it ; and hence, when the ani- 

 mal is dead, the shell always gapes open. The shells are 

 steadied in their places by denticulations adjoining the 

 hinge, but there is no articulation. The ligament is some- 

 times external and sometimes internal : when external it is 

 situated behind the Umbo. The Tridacna is a genus of the 

 family of Chamidaeor Clams, and the species called Tridacna 

 Gigas is the largest of all the molluscs, far exceeding in 

 size and solidity the largest of the bi-valves. These 

 magnificent shells are sometimes used in the churches 

 abroad as holy water basins. There is a pair in the church 

 of St. ^ulpice in Paris, and another in the church of St. 

 Eustache. 



