440 MANUAL OF THE MOLLUSCA. 



»n»7, Nassa, dindiNa.tica. From low-water to the depth of one or two fathoms 

 oa muddy and sandy shores, there are often great meadows of grass-wrack 

 {Zoster a) which afford shelter to numerous shell-fish, and are the haunt of 

 the cuttle-fish and calamary. In tropical seas, the reef-building corals often 

 take the place of sea- weeds, and extend their operations to a depth of about 

 25 fathoms. They cover the bottom with living verdure, on which many of 

 the carnivorous mollusks feed, while some, like Ovulum and Purpura, browse 

 on the flexible Gorgonice. To this zone belong the oyster-banks of our seas, 

 and the pearl-fisheries of the south ; it is richer than any other in animal life, 

 and affords the most highly coloured shells. 



3. Coralline zone. In northern seas the belt of sea-weed that fringes 

 the coast is succeeded by a zone where horny zoophytes abound, and the chief 

 vegetable growth consists of NuUipore which covers rocks and shells with its 

 stony-looking incrustations. This zone extends from 15 or 25, to 35 or 50 

 fathoms, and is inhabited by many of the predacious genera — Buccinum, 

 Fusus, Pleurotoma, Nalica, Aporrhais, Philine, Velntina; and by vegetable 

 feeders, such as Fissurella, Emargimda, PUeopsis, Eulima, and Chemnitzia. 

 The great banks of scallops belong to the shallower part of this region, and 

 many bivalves of the genera Lima, Area, Nucula, Astarte, Venus, Artemis, 

 and Corbula. 



4. Beep-sea Coral-zone. From 50 to 100 fathoms the Nullipore still 

 abounds, and small branching corals to which the Terehratulce adhere. In 

 northern seas the largest corals {Oculina and Primnoa) are found in this 

 zone, and shells are relatively more abundant, owing to the uniformity of 

 temperature at these depths. These deep-water shells are mostly small and 

 destitute of bright colours; but interesting from the circumstances under 

 which they are found, their wide range, and high antiquity. Amongst the 

 characteristic genera are Crania, Thetis, Ne^era, Cryptodon, Yoldia, Benta- 

 liiim, and Scissurella. In the mud brought up from deep water may be often 

 found the shells of Pteropoda, and other mollusca which live at the surface 

 of the sea. In the iEgean Sea there is deep-water within one or two miles 

 of the coast ; but in the British Channel the depth seldom amounts to more 

 than 20—40 fathoms. 



"When registering the results of dredging-operations, it is important to 

 distinguish between dead and living shells, as in the preceding Tables; for 

 almost every species is met with, in the condition of dead shells, at depths 

 far greater than those in which it actually lives. On precipitous coasts the 

 litoral shells fall into deep water, and are mingled with the inhabitants of 

 other zones; currents also may transport dead shells to some distance over 

 the bed of the sea. But the principal agents by wliich so many decayed and 

 broken shells are scattered over the bed of the deep sea, must be the mollusk- 

 eating fishes. Of 140 species of boreal shells described by Dr. Gould (p. 358) 



