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. Psammohia 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 ellijptica^ Solecurtus candiduSj 

 Pandora incequivalvis and its variety ohtusa or pinna^ Chiton 

 Icevis^ Tectura virgineaj Trochus zizyphinuSj Pleurotoma Icevi- 

 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 s&ptemradiatuSj P. opercularis^ Lima hians^ My- 

 tilus AdriaticuSy Isocardia cor^ Astarte sulcata^ Venus exoleta^ 

 V. lincttty Tellina halaustina^ Chiton Hanleyi^ Tectura virgineaj 

 NaticaAlderij Defrancia teres^D. purpurea jSmd 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- 

 phinusj Turritella terehra^ and Eulima hilineata may be men- 

 tioned as examples. 



5. Mollusca inhabiting deep water have consequently a 

 larger supply of oxygen for the aeration of their gills than 

 those which live in shallow water. See my account of Colum- 

 hella halimeti, 



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. Semifossil shells of arctic species (such as Pecten Islandi- 

 cuSj Tellina calcarea, Mya truncata^ var. Uddevallensisj Molleria 

 costulata^ Trochus cinereus^ and Trophon clathratu^) are met 

 with on the sea-bottom at considerable depths, and at some 

 distance from land. The only explanation I can ofler 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. 



