Mr. Gwyn Jeffreys on Dredging among the Hebrides. 389 
ciensis are in this category. All the above (with the exception 
of Buccinum Humphreysianum, 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 Calyptrea) 
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 Kellia family) hatch 
them within the folds of the mantle, whence they are excluded 
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 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 im 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 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 
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. Iam 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 
calearia, Mya truncata, var. Uddevallensis, Trochus cinereus, and 
Astyris Holbolla ; and I have lately, as well as on a former 
