THE HAMMER OYSTERS Genus MALLEUS, Lam.
Shell nearly equivalve, elongated, irregular; dorsal margin prolonged almost at right angles with the valves. A small genus distributed in Chinese and Australian seas.
The Common Hammer Oyster (M. vulgaris, Linn.) is the type, and Lamarck could not but call the genus Malleus, for there was the handle and head of this familiar tool closely imi tated by the valves and elongated wings of the new and unknown mollusk. The coarse, thick valves are drab or black and undu lated. They frequently slant obliquely from the hinge line, and curve decidedly. In early life the shell is like those of Avicula and the byssal cord passes out through a deep sinus in the right valve. Later the extension of the valve in three directions gives it the characteristic form, and the byssal sinus is midway between the two valves, causing an indentation of each. The lining shows
that the body lies in a roundish space near the hinge, attached by a large muscle scar. Length, 6 to 8 inches. Width, 8 to to inches.
Habitat.— China Seas.
The White Hammer Oyster (M. albus, Rve.) differs from the preceding species by being colourless, and of finer and more horny texture, and less rude pattern throughout. Length and width, 6 to to inches.
Habitat.— Philippines.
399 The Wing Shells, Pearl Oysters and Hammer Oysters By a strange chance, the species discovered since the genus was first built up on the characters of M. vulgaris, have, with the exception of M. albus, lacked the peculiar double-winged pro longation at the hinge. The others are headless hammers, with valves of considerable irregularity of form. However, the name stands, describing the largest and most important members.