OF CONCHOLOGY. 27 



sea-bed by geological or cosmical causes, the further diffusion of 

 any marine animals in that direction must necessarily be stopped. 

 An opposite result would doubtless be produced by a sinking and 

 submersion of dry land below the level of the sea, whereby the 

 diffusion of such animals would be greatly facilitated. This ap- 

 pears to have been the fluctuating course of events since the 

 formation of the Coralline Crag, which was probably the cradle 

 or starting-point of our moUuscan fauna — a period long antece- 

 dent to the last glacial epoch, and incalculably far beyond the 

 advent of man, unless his origin is much more remote than it is 

 at present supposed to be. I am not inclined to attribute the 

 northern character of some of the Hebridean mollusca to the per- 

 sistence of what have been called "boreal outliers." Tlie idea 

 savors more of poetry than of philosophy or fact. The boreal or 

 truly arctic species which once flourished in this district have be- 

 come quite extinct, probably in consequence of one of those 

 revolutions above suggested, by which the sea-bed was converted 

 into dry land. These boreal species consist chiefly of Rhyncho- 

 nella psittacea, Pecten Islandicus, Astarte crehricostata or de- 

 pressa, TeUina calea7'ia, Mya truncata, var. UddevaUensis, Tro- 

 chvs chiereus, and Astyris Holhollii ; and I have lately, as well 

 as on a former occasion, dredged them on the coasts of Skye and 

 West Ross, at depths of from 30 to 60 fathoms, or 180 — 360 

 feet. They had a semifossilized appearance. Not one of the 

 above-named species has ever, to the best of my knowledge and 

 belief, been found in a living or recent state in any part of the 

 British seas. All of them occur in post-tertiary or quarternary 

 deposits on the west coast of Scotland, from a few feet above 

 high-water mark* to 320 feet above the present level of the sea.f 

 The greatest subaerial height (320 feet) being added to the great- 

 est submarine depth as above (360 feet), gives an extent of eleva- 

 tion and subsidence equal to 680 feet. But as Pecten Islandicus, 

 for example, now inhabits the Arctic Ocean at depths varying 

 from 5 to 150 fathoms, let us take the average of these depths, 

 viz. 77| fathoms or 465 feet, and add it to the 680 feet. This 

 "would make 1145 feet, and probably represent the height at 

 which the sea-level may be supposed to have stood when P. 

 Islandicus lived on the highest fossiliferous spot noticed by Mr. 

 Watson. The non-fossiliferous boulder-clay, indicating the sim- 

 ultaneous presence of arctic land which was also subject to glacial 



* British Association Report, 1862, Trans. Sect. p. 73: Jeffreys, " On 

 an Ancient Sea-bed and Beach near Fort William, Inverness-shire." 



t Transactions of the Royal Society of Edinburgh, 1864, p. 526 : Rev. 

 R. B. Watson, " On the Great Drift-beds with Shells in the South of 

 Arran." 



