17(i BATHYMETRICAL DISTRIBUTION OF MARINE MOLLUSKvS. 



these currents mollusks may spread into localities distant from 

 their specific metropolis. 



The five bathymetrical zones are : 



1. Littoral. Between tide-marks. 



2. Laminarian. From low water to 15 fathoms. 



3. Coralline. Fi'om 15 to 50 fathoms. 



4. Deep-Sea. From 50 to about 300 fathoms. 



5. Abyssal. From 300 to 3000 fathoms, or more. 



1. The Littoral Zone depends for its depth on the rise and fall 

 of the tide, and for its extent on the form of the shore. The 

 shells of this zone are more limited in their range than those 

 which are protected from the vicissitudes of climate by living at 

 some depth in the sea. The characteristic genera of rocky shores 

 are Littorina, Patella, and Purpura ; of sandy beaches, Cardium, 

 Tellina, Solen, Mactra, J^atica, Donax, Petricola, etc. ; gravelly 

 shores, Mytilus ; and on muddy shores, Lutraria and Pullastra. 

 On rocky coasts are also found many species of Haliotis, 

 Siphonaria, Fissurella, and Trochas ; they occur at various 

 levels, some only at the high-water line, others in a middle zone, 

 or at the verge of low water. C^qijrsea and Conus shelter under 

 coral-blocks, and Cerithium, Terebra, JSTatica, and Pyramidella 

 bury in sand at low water, but may be found by tracing the 

 marks of their long burrows. The sea-shore collector will obtain 

 principally littoral shells ; with occasionally others washed ashore 

 from greater depths ; usually dead and empty. 



2. Laminarian Zone. In this region, when rocky, the tangle 

 (Laminaria) and other sea-weeds form miniature forests, the 

 resort of the vegetable-feeding mollusks — Lacuna, Rissoa, 

 Nacella, Trochus, Aplysia, and various Xudibranchiata. On soft 

 sea-beds bivalves abound and form the prey of Buccinum, Nassa, 

 and Natica. From low-water to the depth of one or two fathoms 

 on muddy and sandy shores, there are often great meadows of 

 grass-wrack (Zostera) which afford shelter to numerous shell- 

 fish, and are the haunt of the cuttle-fish and calamar3^ 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 Grorgoni^. 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 aflfords the 

 most highly colored shells. 



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

 fringes the coast is succeeded by a zone where horny zooph3'^tes 

 abound, and the chief vegetable growth consists of NuUipore, 

 which covers rocks and shells with its stony-looking incrusta- 



