ON DREDGING AMONG THE SHETLAND ISLES. 237 
gration of animals, not the comparative depth of water. Psammobia costu- 
lata and Buccinum undatum are instances in support of this proposition. 
2. Specimens or varieties of the same species are larger in the littoral and 
laminarian zones than in deeper water: e. g. Mactra solida and its variety 
elliptica, Solecurtus candidus, Pandora inequivalvis and its variety obtusa or 
pinna, Chiton levis, Tectura virginet, Trochus zizyphinus, Pleurotoma leevi- 
gata, and Philine aperta. 
3. The size of North-European specimens is usually greater than that of 
South-European specimens of the same species, e. g. Pecten septemradiatus, 
P. opercularis, Lima hians, Mytilus Adriaticus, Isocardia cor, Astarte sulcata, 
Venus exoleta, V.lincta, Tellina balaustina, Chiton Hanley, Tectura virginea, 
Natica Alderi, Defrancia teres, D. purpurea, and Bulla utriculus. 
4, The colour of specimens from the greatest depths is not less vivid than 
of those from shallow water, although each zone has colourless specimens. 
Venus ovata, Trochus zizyphinus, Turritella terebra, and Eulima bilineata may 
be mentioned as examples. This was lately confirmed by a great authority, 
Professor Sars, who has given* numerous instances in illustration of it, founded 
on his son’s dredgings at depths varying from 250 to 300 fathoms among 
the Loffoden Isles. The recent investigations of Dr. W. B. Carpenter and Pro- 
fessor Wyville Thomson in the North Atlantic, by means of the dredge, at 
much greater depths show also that the shells there procured (e. g. Venus ovata 
and Columbella haliceti) were highly coloured_and variegated. In the ‘ Bul- 
letin of the Museum of Comparative Zoology ’ at Harvard College, Cambridge, 
U.S., for 1868, will be found an interesting paper by Count L. F. Pourtales, 
entitled “Contributions to the Fauna of the Gulf-stream at great depths.” 
He says that at the greatest depths which he explored, reaching to 517 fathoms, 
“the prevailing colours are white, pink, sometimes playing into orange, anda 
pale green. Blue was only seen in a small encrusting sponge.” And he 
further remarks that ‘“‘ the deep-sea animals have generally well-developed 
eyes, larger, if anything, than those of their congeners of shallow water.” 
5. Mollusca inhabiting deep water have a larger supply of oxygen for the 
aération of their gills than those which live in shallow water. See my account 
of Columbella haliceti. 
6. The occurrence of the same species in the North Sea and the Mediter- 
ranean results partly from former geological or cosmical conditions, and partly 
from a communication which once existed between the Bay of Biscay and the 
Gulf of Lyons. 
7. Oceanic or floating shells of exotic species are carried northwards by west- 
erly winds, and not directly by the Gulf-stream, which does not reach our coasts. 
8. Land and freshwater mollusca are scarce in Shetland, owing to the 
scantiness of succulent vegetation for their food, and of lime for the con- 
struction of their shells. These are smaller than southern specimens; and 
the same fact is observable with respect to Shetland insects. 
9. Semifossil shells of arctic species (such as Pecten Islandicus, Tellina 
calearia, Mya truncata, var. Uddevallensis, Molleria costulata, Trochus cinereus, 
and Trophon clathratus) are met with on the sea-bottom at considerable 
depths, and at some distance from land. The only explanation I can offer is 
a former elevation of the sea-bed whereon these mollusks lived (and which 
was probably in shallow water), its conversion into dry land, and a subsequent 
subsidence. Perhaps the sea-bed is still sinking. 
10. Species recorded from the Coralline Crag and earlier deposits, and sup- 
posed to be extinct, have now been discovered living in the Shetland seas ; 
e. g. Limopsis aurita, Pleurotoma carinata, and Columbella haliceti. Possibly 
* Vidensk.-Selsk. Forhandlinger for 1868, pp. 27 & 28. 
