14 H. H. Howorth—Traces of a Great Post-Glacial Flood. 
be rare at one part of a clay pit, and within a hundred yards or less, 
and in precisely the same clay, be abundant and characteristic, so 
that a fresh excavation may give a new aspect to the apparent 
grouping of the various forms. Exactly the same method of dis- 
tribution prevails with these fossils from Post-Tertiary beds as with 
the inhabitants of the sea-bottom of the present day. Different 
species have their peculiar individualities of habitat, often within 
very short distances of each other. Solen ensis, e.g. is plentiful 
round Cumbrae in all the banks of muddy sand, but upon one bank 
S. siliqua occurs, and there scarcely an example of S. ensis has yet 
been found, although the banks are as nearly as possible similar in 
composition and not a quarter of a mile from each other. In the 
same way, Pecten Islandicus is abundant in the glacial clay in one 
locality, and will be replaced in the immediate neighbourhood by a 
different characteristic shell. Cytheridea punctillata (to give another 
example) — an abundant fossil form —is plentiful living in Loch 
Fyne, but rare in every other part of the west coast. The collector 
of glacial fossils is thus in exactly the position of the marine 
naturalist—he has to deal with an elevated sea-bottom in which 
many forms had their own especial dwelling-places” (Trans. Geol. 
Soc. of Glasgow, vol. ii. p. 269). 
The fact is, the distribution of the mollusca is in all probability 
ruled by the same laws as that of the land fauna. It depends a great 
deal more on a sufficient supply of suitable food than on merely 
climatic conditions, and when we find Southern and Northern forms 
mixed together in so-called glacial beds, as they have been so clearly 
shown by Mr. Searles Wood to be mixed in a larger degree in the 
beds of the Crag, we ought to feel no more surprise than we do 
when we meet with the remains of the Reindeer and the Hippopotamus 
lying together in the Brick-earths of the Thames Valley, or the Rein- 
deer and the Tiger living together, at this moment, in Manchuria, and 
the Camel and Mountain-sheep in Tibet. This analogy points another 
moral. As we have tried to urge, during the Glacial period proper the 
sea was probably as azoic as the land; when the glacial conditions 
began to mitigate, the land was first occupied by the Reindeer, the 
Marmot, the Lemming, etc., and presently by a fauna requiring more 
genial conditions. The sea must have had precisely the same réle of 
climatic surroundings, and the earlier terrestrial beds are matched by 
the more arctic marine beds of Caithness and of Scandinavia. The two 
sets of marine and subaerial beds were contemporaneous. Of this we 
have remarkable evidence in certain situations where the debris of the 
land-life and of the sea-life have been mingled together. Thus in 
Western Sweden, in the black clays, the following list of shells mark 
the molluscan fauna: Mytilus edulis, Mya truncata, Modiola modiolus, 
Solen ensis, Cyprina islandica, Nucula nucleus, Leda pernula, L. 
caudata, Cardium edule, C. echinatum, C. fasciatum, C. norvegicum, 
Incina borealis, Montacuta bidentata, Isocardia cor, Pecten islandicus, 
P. maximus, P. septemradiatus, P. striatus, Saxicava rugosa, S. arctica, 
Tellina proxima, T. solidula, Astarte elliptica, A. sulcata, A. compressa, 
Thracia villosiuscula, Mactra subtruncata, Tapes pullastra, Venus 
