Geographical Distribution of British Plants. 3l 
neighbouring seas.* But this fauna is by no means rich in 
species and individuals when compared with that of the Crag 
which preceded it, or that of the present coasts which fol- 
lowed it. This difference must arise from the climatological 
conditions in which it lived, conditions not favourable in con- 
sequence of a lower temperature. The fauna of the quater- 
nary molluses of the British Islands has been placed nume- 
rically between the present fauna of Greenland and that of 
the coasts of Massachusetts, although nearer the former, and 
probably very nearly allied to that of the coast of Labrador. 
It is composed of species living in the seas of Great Britain, 
and original natives of the north; of others now confined to 
the more northern latitudes: some appear to be extinct, and 
one or two may have had a southern origin, or rather are 
known only in the Crag of the south of Ireland. The species 
most abundant, and most widely distributed throughout the 
drift, are essentially northern. 
Reverting to the distribution of the molluses on the coasts 
of England (p. 371), a subject which he had discussed in a 
previous memoir,t Mr Forbes distinguishes four zones or re- 
gions: the littoral zone ; that of the laminariz ; that of the 
corallines ; and that of the polypi of the deep seas. Among 
a multitude of interesting details, he shews that the first 
zone is comprised between the sea when at the highest and 
when at the lowest; the second between the low water and a 
depth of 27 metres ; the third extends from 27 to 90 metres ; 
and the fourth from 90 to 180 metres, and beyond it. With 
these data, he first determines the existence of the first zone 
in the quaternary deposits, and then proves that the latter 
are in no respect deposited under a depth of water which 
could reach the fourth. The greater number of the fossils 
must have lived in cold waters, not of great depth, and be- 
longing to the three first zones. The arctic characters of 
this fauna are not, therefore, the result of it having lived at 
* See likewise Agassiz, Coquilles fossiles d’Angleterre identiques avec des 
especes vivantes, Verh. d. Schweitz naturf Ges in Zwich. 1841, p. 63. 
t+ Edinburgh Academic Annual for 1840. 
