DISTRIBUTION IN DEPTH. 151 
The distribution of the Mollusca in Depth has been investigated 
by MM. Audouin and Milne-Edwards, M. Sars, and Professor 
i. Forbes. By these observers the sea-bed is divided into four 
principal regions :— 
1. The Littoral zone, or tract between tide marks. 
2. The Laminarian zone, from low water to 15 fathoms. 
3. The Coralline zone, from 15 to 50 fathoms. . 
4, The deep-sea coral zone, 50 to 100 fathoms or more. 
1. Lhe 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.* In Europe the characteristic genera 
of rocky shores are Littorina, Patella, and Purpura; of sandy 
beaches, Cardiwm, Tellina, Solen; gravelly shores, Mytilus ; 
and on muddy shores, Lutraria and Pullastra. On rocky coasts 
are also found many species of Haliotis, Siphonaria, Fissurella, 
and Trochus; they occur at various levels, some only at the 
high-water line, others in a middle zone, or at the verge of 
low-water. Cyprea and Conus shelter under coral-blocks, and 
Cerithium, Terebra, Natica, and Pyramidella bury in sand at low 
water, but may be found by tracing the marks of their long 
burrows. (Macgillivray.) 
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 Nudibranchiata. On soft sea-beds 
bivalyesabound and form the prey of Buccinwm, 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 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 
carniyorous mollusks feed, while some, like Ovulwm and Purpura, 
browse on the flexible Gorgonie. 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. 
Some of the littoral shells, like Purpura lapillus and Littorina rudis, have no 
free-swimming larval condition, but commence life as crawlers, with a well-developed 
shell. Their habits are sluggish, and tnetr diffusion by ordinary means must be 
exceedingly slow. 
