THE FURBELOWED CLAMS FAMILY TRIDACNIDAE. Shells equivalve, regular, truncated in front, very hard and heavy; ligament external; valves strongly ribbed from the hinge outward, ribs frilled, with wavy concentric hinge with two interlocking teeth; mantle closed, but pedal opening large; foot finger-like, with byssal groove; shell muscle large, central.
Genus TRIDACNA, Brug.
The Tridacnce or Furbelowed Clams, one of which, T. gigas, is the largest of all mollusks, live in beds of some extent in lagunes among coral reefs, among the islands of the Eastern and Pacific seas. The shell is generally white, sometimes tinged with red and saffron or brown-yellow, but the animal is brilliantly coloured. M. Quay describes the beautiful iridescent glare of blue, violet, and yellow, variegated with fantastic markings, that is presented by these sub-marine parterres as seen through the clear blue water; and Mr. Cuming speaks with enthusiasm of passing over a mass of them nearly a mile in extent, which resembled nothing so much as a beautiful bed of tulips.— Reeve.
The Scaly Tridacna (T. squamosa, Lam.) has symmetri cal valves, its rounded ribs adorned with erected frills "that are developed throughout with amplitude and precision." Fine can cellation adorns the grooves. The white substance of these massive shells is tinged or concentrically streaked outside with yellow or red. Diameter, 4 to 8 inches.
Habitat.— Moluccas.
The Giant Tridacna (T. gigas, Lam.) attains the enormous weight of six to seven hundredweight, its massive bulk meas uring two to three feet across. One sees the single valves used as benetiers at the doors of Catholic churches. The animals in
habiting these gigantic shells weigh upward of twenty-five pounds, and are described as very good to eat. The natives of the Caroline Islands hew axe heads out of the thickest portions 363 The Furbelowed Clams of these shells, whose hardness is remarkable. So far has the process of calcification proceeded that scarcely a trace of animal matter remains in an adult shell. Out of the open lunule (the depression in front of the hinge) the foot projects and from the gland a powerful tendinous byssal cord is spun. By this the mollusk is anchored to coral rocks.
Habitat.— Indian and Pacific Oceans.
The Serrated Tridacna (T. serrifera, Lam.) has the sym metry of T. squamosa, and the few broad ribs of T. gigas; but the scales are reduced to lines of sharpened points following the ribs out but a short way from the umbones, and always more distinct on the anterior region. The whole surface is crossed by stria tions, radiating and concentric, which are stronger in the broad interstices than on the ridges. The white surface has a yellow tinge. Diameter, 3 to 4 inches.
Habitat.—Moluccas.
Three smaller species occur in the Philippines, all obliquely elongated, and well ruffled, so that one can know them at a glance as belonging to the genus.