OF CONCHOLOOY. 1G3 



much sought after — consists of Norwegian fir trees, often with 

 their roots, and drilled by the same species of Teredo (T. mego- 

 tara or nana) that attacks piles and fixed wood-work, as well 

 as boats equally in the harbors of East Shetland and Norway. 

 Besides, another consideration must not be lost sight of, viz., 

 that the Mediterranean fauna is quite independent of the 

 Gulf Stream; and a glance at Maury's chart will show that 

 the direction of its course, or of the 'drift' which may have 

 been mistaken for it, off the western coasts of Europe and 

 Africa (including the entrance to the Straits of Gibraltar), is 

 entirely southward, and forms, in fact, the return current. The 

 present distribution of marine life in the European seas must 

 be traced in some other way, and with reference to geological 

 conditions. Palaeontologists are well aware that many kinds 

 of Mollusca which still inhabit the Mediterranean, but not our 

 seas, left their remains on the area that now constitutes the 

 eastern coasts Of England, perhaps at a period long antecedent 

 to the origin of the Gulf Stream. Several species of Mollusca, 

 which may be termed southern forms, likewise occur on the 

 Dogger Bank and the coasts of Yorkshire and Northumber- 

 land; and among them may be enumerated Trochus millegra- 

 nus, Scalaria Turtoni, Natica sordida, Murex erinaceous, De- 

 francia brachystoma, and Pleurotoma teres. . . ." 



" It seems to me as if shells belonging to the same species, 

 that are common to the littoral and deep-water zones, attain a 

 greater size and thickness in the former than in the latter 

 habitat. Such are Venus gallina, Tellina fabula, Mactra solida 

 (compared with its variety elliptica), Teetura virginea, Rissoa 

 Alder i, Trochus zizyphinus, T. ciaerarius, T. tumidus, and Buc- 

 cinum undatum (compared with its variety Zetlandica). Mr. 

 Jordan informs me that he has observed the same difference 

 with regard to specimens of Pandora insequivalvis and its 

 variety obtusa, Teetura virginea, and Chiton disci-epans, which 

 he has lately taken on the shore and dredged off the Channel 

 Isles. More extensive observations are unquestionably de- 

 sirable, if not necessary, before this proposition can be substan- 

 tiated ; but it has been abundantly proved by the researches 

 of Dr. Davy, Forch hammer, and Bishof, that the quantity of 

 carbonate of lime held in solution by sea-water, and from which 

 shells are secreted, occurs chiefly along coast-lines, being dc 

 rived from terrestrial sources, and brought down to the sea by 

 rivers, streams, and the washings of rain and waves. This 

 would give a reason for littoral shells being more solid than 

 those from deep water ; and possibly the greater abundance of 

 food in the former than in the latter case might account for 

 the increase of bulk. 



