304 Mr. J. Gwyn Jeffreys on Dredging 
littoral and submarine ; the nature of the habitat and the sup- 
ply of food influence the residence and migration of animals, 
not the comparative depth of water. Psammobia costulata 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 virginea, Trochus zizyphinus, Pleurotoma levi- 
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, My- 
tilus Adriaticus, Isocardia cor, Astarte sulcata, Venus exoleta, 
V. lincta, Tellina balaustina, Chiton Hanleyt, Tectura virginea, 
Natica Alder, 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 zizy- 
phinus, Turritella terebra, and Kulima bilineata may be men- 
tioned as examples. 
5. Mollusca inhabiting deep water have consequently a 
larger supply of oxygen for the aération of their gills than 
those which live in shallow water. See my account of Colwm- 
bella halicett. 
6. The occurrence of the same species in the North Sea and 
the Mediterranean 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. Exotic and oceanic shells 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 construction of their shells. These are 
smaller than southern specimens; and the same fact is obser- 
vable with respect to Shetland insects. 
9. Semitfossil shells of arctic species (such as Pecten [slandi- 
cus, Tellina calcarea, Mya truncata, var. Uddevallensis, Méllerva 
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), and its conversion 
into dry land, and a subsequent subsidence. Perhaps the sea- 
bed is still sinking. 
