230 NATURAL HISTORY. 



abundant, the largest C. tridentata. Clio cuspidata, with a fretted shell, whose ornament reminds one of 

 some of the fossil genera, is perhaps the species most frequently seen on the surface, and the one which 

 shows the iridescent colouring with the greatest brilliancy. The several species of Styliola, much smaller 

 than the others, are much more numerous, and sometimes throng the towing-net with their glassy needles. 

 Styliola subulata, S. acicula, and *S'. virgula are in immense abundance, and very generally distributed. 

 Some of these species sometimes reach the coast of Britain, but an indraught of northern water, which 

 includes the British Islands in a fork, keeps out these oceanic things from our shores. If the Britisii 

 naturalist, to whom these things are usually unknown in a living state, will only push his towing-net 

 work by a tug steamer, or his own or a friend's yacht, forty or fifty miles from the West Coast 

 of Scotland or Ireland, he will get beyond the Arctic water, and will wonder, as I did, at the 

 new animal world, in the shape of Pteropoda, Heteropoda, Siphonophora, and, above all, Polycystin^ 

 and Acanthometrina, in all their wonderful varieties of form and sculpture, which will suddenly burst 

 upon him. 



" The Pteropoda extend far to the northward ; one, Limacina helicina, with a delicate but very 

 elegant spiral shell, and another, Clione bwealis, which belongs to the shell-less subdivision, are 

 frequently seen by Arctic voyagers in such numbers that they actually colour the surface of the sea 

 in patches of many square miles in extent, and they are said to form a considerable item in the food 

 of the Greenland Whale, which strains them out of the water as it passes through his mouth with 

 his whalebone sieve. I have dwelt on this little group because their history is not very familiar, and 

 because, small as they are, they play by no means an unimportant part in some of the recent 

 geological processes of reconstruction. " * 



CHAPTER IV. 



THE CONCHIFERA. 



Class IV., CONCHIFERA Bivalve Shells Their Sedentary or Burrowing Habits Structure of Bivalve Shells - 

 Anatomy of Animal Muscles, Mantle, Gills, &c. Family L OSTREID.E Oysters Their Economic Value Frank 

 Bucklancl on the Oyster The "Points "of an Oyster Mode of Cultivation The Young Oysters Their Enemies 

 Their Sensitiveness to Cold Ancient Shell Relics 2. ANOMIAD.E 3. PECTINID.K- Scallop Shells The St. 

 James's Shell Vivacity of Young Pecten The Spondylus, or Thorny Oyster Value of Bivalve Shells 4. 

 AVICULID-E Pearl Oysters Use of Shells Value of Pearls Pearl-fishery, Ceylon The Divers THE GREAT PINNA 

 5. MYTILID.E Mussels A Bridge Preserved by Mussels from Destruction Boring Shells, Lithodomi (>. ARCADE 

 7. TKIGONIAD^E Trigonia 8. UNIONID^E River Mussels Pearl Mussels 9. CHAMID.E 10. TRiDACNiD.fi Giant 

 Clams 11. CARDIAD^E 12. LUCINID^E 13. CYCLADID.E 14. ASTARTID.E 15. CYPRINID.E -16. VENEKID^E 17. 

 MACTRID* 18. TELLINID^E 19. SOLENID.E " Razor-shells " 20. MYACID.E 21. ANATINID.E 22. GASTROCH^ENID^E 

 Stone-borers 23. PHOLADID.E Wood-borers The Ship-worm. 



CLASS IV. CONCHIFERA.r 



WE have already glanced at three of the great subdivisions of the Molluscan kingdom 

 namely, the Cuttle-fishes (Cephalopoda), the Snails (Gasteropoda), and the Pteropods. Let us 

 now take a survey of a fourth group, the Conchifera, or " shell-bearing " Mollusca, better 

 known as bivalves, from the fact that the majority are enclosed within a pair of shells united 

 by a hinge, of which the Oyster, Mussel, Cockle, and Scallop are familiar examples. They are 

 never found living on the land, as Snails and Slugs are able to do; and although, owing to their 

 closely-fitting shells, the Dreissena, the Oyster and Mussel, and the fresh-water Cyclas are able 

 to survive exposure for some time, yet as a whole the bivalves are all aquatic, and, with a 

 few exceptions, are all inhabitants of the sea. They occur on the shores of every land in 

 all climates, and are met with from low water-mark to a depth of many hundred fathoms. 



* Sir Wyville Thomson's " Voyage of H.M.S. Challenger," Vol. I., p. 123. 

 f Latin, concha, a shell, and fero, I bear. 



