(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. [Illustration: 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.)]