Some shells are frequently found reversed, among others, Pyrula perverse, many species of pupa, and the entire genera clausilia phyla, and triphoris ; also the whelk and the garden snail are sometimes reversed, and Bulimus citrinus is as often sinistral as dextral. • hijurious.—There are several boring-shells; the ship-worm or teredo and some allied genera per forate timber, whilst the pholas bores into stones, chalk, clay. A piece of serpentine found on the Madras beach was bored with thepholas. The pholas shell is rough like a file. Other boring shells are lithodomus, gastrocluena, saxicava, and ungulina.
The Teredo navalis is ordinarily a foot long, sometimes 2} feet. It destroys soft wood rapidly, and teak and oak do not escape.
Teredo corniformis, Lam., is found burrowing in the husks of cocoanuts and other woody fruits floating in tropical seas.
The bivalves obtain their food by filtering water through their gills. They do not feed upon prey caught between their valves. Whatever particles the current brings are collected on the surface of the breathing organ and conveyed to the mouth. It is in this manner that they help to remove the impurities of turbid water.
Some of the gasteropods attach themselves by glutinous threads; litiopa and rissoa parva anchor themselves to seaweeds, and cerithidea and Indian land-snail, Cyclostoma suspensum, suspend them selves. The mussel, pearl oyster, and other of the bivalves habitually spin a byssus, by which they attach themselves to objects. This they can detach and renew, and the mussel is essentially migratory. The byssus of the pinna of the Mediter ranean attains to If feet in length, and at Palermo has been woven into gloves and stockings as'a fancy work.
The injury caused by the teredo has been noticed above. Nearly all the laud molluscs are vegetable feeders. They commit great ravages on the crops of the farmers, particularly on the pea tribe and cabbage tribe ; but they hold white mustard in abhorrence, and they fast or shift their quarters while that crop is on the ground. Snails are • destroyed by salt, but dilute lime-water and very weak alkaline solutions are still more fatal to them. Slugs feed chiefly on decaying vegetable and animal matter. One of them, the Limax noctilucus, Fer., of Teneriffe, has a luminous pore in the posterior border of the mantle.
The mouth of the cephalopoda has two strong horny mandibles, something like the beak of a parrot, and it is surrounded by long fleshy arms, called tentacles, provided with numerous suckers, by means of which the animal grasps tightly what ever comes in its way. In some of •the cephalo pods the tentacles are long and powerful. Banks and Solander, in Cook's first voyage, met with a dead cephalopod in the Pacific, which was esti mated to have been 6 feet long when perfect. It was the Enoploteuthis unguiculata ; an arm of it is in the London College of Surgeons Museum.
Some species of loligo or pen-fish have been seen to leap out of the sea like the flying fish.
Fossils.—The window-shell, Placuna placenta, is at present living in the China seas, but is found fossil in abundance all round the coasts of India, from Sind to Singapore, and in the Peninsula of India and in China is largely used as a substitute for window glass. Of the genera and species of shells discovered in the black clay underlying Madras, the chief are :— Retells. A. granola. Cardita.
Ranella tubercu- Anemia. Placuna.
lata. Natica helvacea. Venus.
Cerithium microp- N. maculosa. Tapes ramosa.
tern. N. mamilla. Dona' scortum.
C. palustre. Purpura carinifera Mactra. C. telescopium. Oliva utriculus. Meroe.
Turritella. 0. irisans. • Cytherea. Eburna spirata. Nassa crenulata. Sanguinolaria Bullia vittata. N. clathrata. diphos.
Ampullaria N. Jacksonianum. Tellina.
globosa. N. thirsites Nucula.
Solarium. Ostrea. Pullastra.
Area disparilis. Artemis. Balanus.
The Pecten Jacobmus or St. James' shell was worn by the former pilgrims to the Holy Land, and became the badge of several Orders of knight hood.
Amussium Japonicum is a large scallop of Japan. The Japanese call it Tsuki-hi-kai, and the sun and moon shell, from its presenting a yellow disc on one side and a white one on the other.
The genus Pecten or scollop shell possesses greater power of locomotion than most bivalves. The movements of the lima or file shell, when in the watei, are graceful, the two valves being used as fins, by means of which it swims with consider able rapidity ; the shell is less curved than the scallop, and generally white ; the valves do not entirely close.
Living. — The following are genera of the salt - water mollusca living in all the 'eastern seas :—.