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 incequivalvis and its variety obtusa or 

 pinna, Chiton Icevis, Tectura viryinea, Trochus zizypliinus, 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 septemradiatus, 

 P. opercularis, Lima hians, Mytilus Adriaticus, Isocardia cor, Astarte sidcata, 

 Venus eocoleta, V. lincta, Tellina balaustina, Chiton Hanleyi, 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 lialiceeti) 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, and a 

 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 

 aeration of their gills than those which live in shallow water. See my account 

 of Columbella haliceeti. 



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 Avith respect to Shetland insects. 



9. Semifossil shells of arctic species (such as Pecten Islandicus, Tellina 

 calcaria, 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 lialiaeeti. Possibly 



* Vidensk.-Selsk. Forhandlinger for 1868, pp. 27 & 28. 



