VARIATION OF TURPURA LAPILLUS 



91 



is an exceedingly varialjle species, and in many cases tlie 

 variations may be shown to bear a direct relation to the manner 

 of life (Fig. 35). Forms occurring in very exposed situations, 

 e.(j. 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 Ijeing 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 l)y the specimens from Eobin 

 Hood's Bay (13, 14). Had these specimens occurred alone, or had 

 they been brought from some distant and unexplored region, 

 they must inevital)ly have been described as two distinct species. 

 ]\Ir. AV. Bateson has made ^ some observations on the shells 

 of Cardium edide taken from a series of terraces on the border 



1 2 3 4 



Fig. 3G. — Valves of Cardium edulc from the four ujiper terraces of Sliumish Kul, a dry 

 salt lake adjacent to the Aral Sea. (After Bateson. ) 



of certain salt lakes which once formed a portion of the Sea of 

 Aral. As these lakes gradually became dry, the water they 

 contained iDecame Salter, and thus the successive layers of dead 

 shells deposited on their Ijorders form an interesting record of 

 the progressive variation of this species under conditions wliich, 

 in one respect at h^ast, can be clearly appreciated. At the same 

 time the diminishing volume of water, and the increasing average 

 temperature, would not be witliout their effect. It was found 



1 ridl. Trans. 1889, vol. 180 B, p. 297. A somewhat similar case (the celebrated 

 Steiuheim scries of Plaitorhis) is dealt with by Ililgendorf, MB. Akad. Bcrl. 1866, 

 p. -IT-i ; and Hyatt, Froc. Ainer. Ass. Sc. xxix. p. 527. 



