Fig. 35.—19 specimens of Purpura lapillus L., Great Britain, illustrating variation.
(1) Felixstowe, sheltered coast; (2), (3) Newquay, on veined and coloured rock; (4) Herm, rather exposed; (5) Solent, very sheltered; (6) Land’s End, exposed rocks, small food supply; (7) Scilly, exposed rocks, fair food supply; (8) St. Leonards, flat mussel beds at extreme low water; (9) Robin Hood’s Bay, sheltered under boulders, good food supply; (10) Rhoscolyn, on oyster bed, 4–7 fath. (Macandrew); (11) Guernsey, rather exposed rocks; (12) Estuary of Conway, very sheltered, abundant food supply; (13), (14) Robin Hood’s Bay, very exposed rocks, poor food supply; (14) slightly monstrous; (15), (16), (17) Morthoe, rather exposed rocks, but abundant food supply; (18) St. Bride’s Bay; (19) L. Swilly, sheltered, but small food supply. All from the author’s collection, except (10).
The common dog-whelk (Purpura lapillus) of our own coasts is an exceedingly variable species, and in many cases the variations may be shown to bear a direct relation to the manner of life (Fig. [35]). Forms occurring in very exposed situations, e.g. Land’s End, outer rocks of the Scilly Is., coasts of N. Devon and Yorkshire, are stunted, with a short spire and relatively large mouth, the latter being developed in order to increase the power of adherence to the rock and consequently of resistance to wave force. On the other hand, shells occurring in sheltered situations, estuaries, narrow straits, or even on open coasts where there is plenty of shelter from the waves, are comparatively of great size, with a well-developed, sometimes produced spire, and a mouth small in proportion to the area of shell surface. In the accompanying figure, the specimens from the Conway estuary and the Solent (12, 5) well illustrate this latter form of shell, while that from exposed rocks is illustrated by the specimens from Robin Hood’s Bay (13, 14). Had these specimens occurred alone, or had they been brought from some distant and unexplored region, they must inevitably have been described as two distinct species.
Fig. 36.—Valves of Cardium edule from the four upper terraces of Shumish Kul, a dry salt lake adjacent to the Aral Sea. (After Bateson.)
Mr. W. Bateson has made[195] some observations on the shells of Cardium edule taken from a series of terraces on the border of certain salt lakes which once formed a portion of the Sea of Aral. As these lakes gradually became dry, the water they contained became salter, and thus the successive layers of dead shells deposited on their borders form an interesting record of the progressive variation of this species under conditions which, in one respect at least, can be clearly appreciated. At the same time the diminishing volume of water, and the increasing average temperature, would not be without their effect. It was found that the principal changes were as follows: the thickness, and consequently the weight, of the shells became diminished, the size of the beaks was reduced, the shell became highly coloured, and diminished considerably in size, and the breadth of the shells increased in proportion to their length (Fig. [36]). Shells of the same species of Cardium, occurring in Lake Mareotis, were found to exhibit very similar variations as regards colour, size, shape, and thickness.
Unio pictorum var. compressa occurs near Norwich at two similar localities six or seven miles distant from one another, under circumstances which tend to show that similar conditions have produced similar results. The form occurs where the river, by bending sharply in horse-shoe shape, causes the current to rush across to the opposite side and form an eddy near the bank on the outside of the bend. Just at the edge of the sharp current next the eddy the shells are found, the peculiar form being probably due to the current continually washing away the soft particles of mud and compelling the shell to elongate itself in order to keep partly buried at the bottom.[196]
The rivers Ouse and Foss, which unite just below York, are rivers of strikingly different character, the Ouse being deep, rapid, with a bare, stony bottom, and little vegetable growth, and receiving a good deal of drainage, while the Foss is shallow, slow, muddy, full of weeds and with very little drainage. In the Foss, fine specimens of Anodonta anatina occur, lustrous, with beautifully rayed shells. A few yards off, in the Ouse, the same species of Anodonta is dull brown in colour, its interior clouded, the beaks and epidermis often deeply eroded. Precisely the same contrast is shown in specimens of Unio tumidus, taken from the same rivers, Ouse specimens being also slightly curved in form. Just above Yearsley Lock in the Foss, Unio tumidus occurs, but always dwarfed and malformed, a result probably due to the effect of rapidly running water upon a species accustomed to live in still water.[197] Simroth records the occurrence of remarkably distorted varieties in two species of Aetheria which lived in swift falls of the River Congo.[198]
A variety of Limnaea peregra with a short spire and rather strong, stoutly built shell occurs in Lakes Windermere, Derwentwater, and Llyn-y-van-fach. It lives adhering to stones in places where there are very few weeds, its shape enabling it to withstand the surf of these large lakes, to which the ordinary form would probably succumb.[199]