THE PERNAS Genus PERNA, Brug.
Shell nearly equivalve, squarish, flattened, right valve with byssal sinus; hinge broad, crossed by a series of ligamental grooves; umbones small, often terminal and hooked. A small genus with variable forms in tropical seas, chiefly eastern.
The Saddle Perna (P. ephippium, Linn.) is well named from its general outline. The coppery-red surface is covered with wavy laminations that seem to have fused and become obsolete. The anterior end has a deep concavity. The umbones form a hook. Diameter, 4 to 5 inches.
Habitat.— Honduras.
The Purse Perna (P. marsupium, Lam.) has much the same form as the pearl oyster. It is a small species from the Philippines.
398 The Wing Shells, Pearl Oysters and Hammer Oysters The Pod Perna (P. legumen, Rve.) is a small quadrangular white shell, three times as long as it is wide.
The Carpenters' Square (P. isognonum, Rve.) is a well known representative of this genus. The elongation of the body of the shell seems to shorten the wing, and vice versa. The angle formed is close to ninety degrees. Colour, dark purple. Length, 6 to 10 inches.
Habitat.— Philippines.
The Bat Perna (P. vespertilio, Rve.) strikingly imitates the shape of a bat's wing. It is yellowish white, tinged with purple. Length, 6 to 8 inches., Habitat.— Bay of Manila.