Mr. Gwyn Jeffreys on Dredging among the Hebrides. 389 



ciensis are in this category. All the above (with the exception 

 of Buccinum Humphrey sianum, which inhabits Shetland and the 

 coasts of county Cork) are met with on the Dogger bank ; and 

 the first two are fossil in the Clyde beds. Six out of the seven 

 being univalves, I would venture to surmise that their non-exist- 

 ence in the western seas of Scotland may have arisen from the 

 circumstance that the diffusion of univalves is slower than that 

 of bivalves. The spawn of the former is attached to the spot 

 where it is shed, or in a few cases (e. g. Capulus and Calyptraa) 

 it is hatched within the shell of its sedentary parent ; so that the 

 fry forms 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 bivalves. These shed their 

 ova into the water, or else (as in some of the iiTeZ/m family) hatch 

 them within the folds of the mantle, whence they are excluded 

 on .irriving 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 may be involuntarily transported 

 by tidal and oceanic currents. Time is the only element neces- 

 sary for their widest dispersion over the adjacent seas, if no 

 barrier 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 upheaval and drying-up of the neighbouring sea-bed by 

 geological or cosmical causes, the further diffusion of any marine 

 animals in that direction must necessarily be stopped. An 

 opposite lesult 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 

 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 molluscan 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 north- 

 ern character of some of the Hebridean mollusca to the persist- 

 ence of what have been called " boreal outliers." The idea savours 

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

 arctic species which once flourished in this district have become 

 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 Rhynchonella psit- 

 tacea,Pecten Islandicus, Astarte crebricostata or depressa, Tellina 

 calcaria, Mya truncata, var. Uddevallensis, Trochtts cinereus, and 

 Astyris HolboUii ; and I have lately, as well as on a former 



