188 REPORT— 1866. 



valves is slower than that of bivalves. The spawn of the former is attached 

 to the spot Avhere it is shed, or in a few cases (e. g. C'cqnthis and Caljiptra'a) 

 it is hutched within the shell of its sedentary parent ; so that the fry form 

 a colony, and need not roam to any distance, provided their station yields a 

 sufficient supply of food and has the other requisites of habitability. Not so 

 with hivalves. These shed their ova into the water, or else (as in some of the 

 KolUa family) hatch them within the folds of the mantle, whence they are ex- 

 cluded on arriving at maturity. Their fry swim freely and rapidly by means of 

 numerous encircling cilia. The metamorphic state lasts many hours. During 

 that period they can voluntarily traverse considerable distances, or they maj- 

 be involuntarily transported by tidal and oceanic currents. Time is the only 

 element necessary for their widest dispersion over the adjacent seas, if no bar- 

 rier intervenes. Should, however, such an obstacle present itself, whether in 

 the shape of previously existing dry land, like that which separates the North 

 Sea from the Atlantic, or from an iiphcaval and drying-iip of the neighbour- 

 ing 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, whercljy the diffusion of such animals would bo 

 greatly facilitated. This appears 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 antecedent 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 persistence of wliat have been called " boreal outliers.'' The idea 

 savours more of poetry than of philosophy or fact. The boreal or truly 

 arctic species whicli once flourished in this district have become quite ex- 

 tinct, probably in consequence of one of those revolutions above suggested, by 

 Avhich the sea-bed was converted into dry land. These boreal species consist 

 chiefly of BliynchoneUa psittacea, Pecten Islandlcus, Astarte crebi-lcosiata or 

 depressa, Tellina calcaria, Mi/a tnincata,\ax. Uddevallensis, Trochus cinereiis, 

 and Asti/ris IloIhbUii. ; and I have lately, as well as on a former occasion, 

 dredged them on the coasts of Skyo and West Eoss, 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-tertiarj' or quaternary deposits on the west coast of 

 Scotland, from a few feet above high -water mark* to 320 feet above the present 

 level of the seaf. The greatest subaerial height (320 feet), being added to 

 the greatest submarine depth as above (360 feet), gives an extent of elevation 

 and subsidence equal to 680 feet. But as Pecicn 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. 77g 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 simultaneous presence of 

 arctic land which was also subject to glacial conditions, is stated bj' Mr. 



" British Association Ecport, 1862, Trans. Spc-t. p. 70 : Jeffreys " On an Ancient Sea- 

 bed and Bead] near Fort William, Inverness-sliire." 



t Transactions of the Koyal Society of Edinburgh. 1864, p. 526: Kev.E. B.Watson, 

 " On the Great Drift-beds with Shells in the gouth of Arrau."' 



