358 Mr. R. M‘Andrew on Mr. Jeffreys’s last Dredging Report. 
by Risso and subsequent authors. There are two principal 
zones, the littoral and the submarine; the nature of habitat 
and supply of food influence the residence and migration of 
animals, not the depth of water. Psammobia costulata and 
Buceinum undatum are instances in support of this proposi- 
tion.’ 
Now a natural inference from this would be that, excepting 
those which are littoral, the species inhabit all depths indif- 
ferently, which I know it could never have been Mr. Jeftreys’s 
intention to imply. Admitting that “the nature of the habitat 
and supply of food influence the residence of animals,” it is 
evident that these must vary to meet the requirements of dif- 
ferent species, also that depth is not only itself an important 
element in the nature of a habitat, but must have considerable 
influence on the food of Mollusca. For instance, the Lami- 
naria does not grow below 15 or 20 fathoms at the most; 
several species of Mollusca are entirely dependent upon Lami- 
narta for food, and consequently are strictly confined to its 
zone. In most cases the conditions cannot be so easily de- 
fined; but it is nevertheless a fact that most, if not all, the 
species of Mollusca have their maximum of development at a 
particular depth—and that while some enjoy a considerable 
bathymetrical range, others are confined within comparatively 
narrow limits—and that, in consequence of the great interest 
attached to vertical distribution, it has been found convenient 
to institute zones of depth as well as geographical provinces. 
According to Forbes, there exist in the eastern Mediterra- 
nean eight well-marked regions of depth, each characterized 
by its peculiar fauna. As this conclusion was arrived at after 
some eighteen months of research by no ordinary observer, I 
consider that it is entitled to respect until the data upon which 
it is founded, set forth in the Report on the Augean Inverte- 
brata, shall have been proved to be erroneous. It does not fol- 
low that observations made in the Augean Nea are of universal 
application, or that different conditions in the ocean or other 
seas may not require a modification in the number or extent 
of the zones. Where the action of the tide is considerable, 
there are, in fact, an upper and a lower littoral zone—some 
species of Mollusca as well as of marine plants being found 
even beyond the reach of ordinary tides, while others are not 
to be met with much above the lowest water-mark. It is to 
be noted that the same species often frequent different depths 
in different seas; and in these cases it is generally where the 
climate and other conditions are most favourable to their exis- 
tence and multiplication that they inhabit the shallowest 
water. Several of the rare Shetland species, which might 
