1. Littoral zone, not exceeding 2 fathoms in depth, its ground various.
2. Zone extending from 2 to 10 fathoms; mud or sand.
3. Zone extending from 10 to 20 fathoms; gravelly or sandy.
4. Zone extending from 20 to 35 fathoms; mud and gravel.
5. Zone extending from 35 to 55 fathoms; nullipore and shells.
6. Zone extending from 55 to 79 fathoms; nullipore.
7. Zone extending from 80 to 105 fathoms; nullipore chiefly.
8. Zone extending from 105 to 230 fathoms; yellow mud.
In this sea occurred 3 Cephalopoda, 8 Pteropoda, 10 Nucleobranchiata, 16 Nudibranchiate Gasteropoda, 60 Inferobranchiate, Tectibranchiate, Scutibranchiate, Cyclobranchiate and Cirrhobranchiate Gasteropoda; 1 Pulmoniferous Gasteropod; 190 Pectinibranchiate Gasteropoda; 8 Brachiopoda, 115 Dimyarian Lamellibranchiata, and 28 Monomyarian Lamellibranchiata, 17 Tunicata; 57 Arachnodermata, 47 Echinodermata; of Zoophytes not many species; of sponges 20.
To all these Professor Forbes assigns the characteristic regions of depth, nature of favourite ground, and geographical distribution. The most positive and determining influence is that of the ground or sea-bottom. "According as rock, sand, mud, weedy or gravelly ground prevails, so will the numbers of the several genera and species vary. The presence of the sponges of commerce often depends on the rising up of peaks of rock in the deep water near the coast. As mud forms the most extensive portion of the bottom of the sea, bivalve mollusca abound more individually, though not specifically, than univalves. As the deepest sea bottom is of fine mud, it is there the delicate shells of pteropoda and nucleobranchiata are preserved. Where the bottom is weedy, we find the naked mollusca more numerous than elsewhere; where rocky, the strong shelled gasteropoda and active cephalopoda. Few species, either of mollusca or radiata, inhabit all bottoms indifferently."
