We shall now say a few words on the depth of the sea, or ocean, in which Mollusca are found.
The observations of Milne Edwards, Audonin, and Professor Edward Forbes, have led to the division of the sea into four zones.
The deep sea Coral zone, from fifty to one hundred fathoms; the Coralline zone from fifteen to fifty fathoms; the Laminarian zone, which stretches from fifteen fathoms to low water; and the Littoral zone, between high and low water marks. The great stronghold of Crania, Thetis, Neæra, Yoldia, Dentalium, and Scissurella, is in the deep sea Coral zone; while Buccinum, Fusus, Pleurotoma, Natica, Aporrhais, Philine, and Velutina, which are among the most ravenous and predatory of molluscs, are found in the Coralline zone. They attack the bivalves, whose shells among the relics of former seas, as in those of the present, show evidence of an assault and a murder.
The principal genera of the Laminarian zone are the Nudibranchiata, Aplysia, Trochus, Nacella, Rissoa, and Lacuna, which feed so much on the seaweed of this region.
The Littoral zone, which being accessible as the tide recedes, is best known, affords Cardium, Mytilus, Tellina, Solen, Trochus, Patella, Littorina, and Purpura; or in plain English, cockles, mussels, razor-fish, limpets, periwinkles and tingles;—species which are the first to attract our attention, and which are so much used for food.
CHAPTER XVII.
THE CRUSTACEA.
"Multa tamen lætus tristia pontus habet."