i8 



The interpretation of the causes leading to the development 

 of local races in Thais by the earlier authors is well expressed 

 in the excellent account given by Cooke (1895: p. 91) where 

 he states:- "The common dog-whelk {Purpura lapillus) of our 

 coast 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 occurrmg in very exposed situ- 

 ations, e. g. Land's End, the outer rocks of the Scilly Is., 

 coasts of N. Devon and Yorkshire, are stunfed, with a short 

 spire and relatively large mouth, the latter being developed in 

 order to increase the power of adherence to the rock and con- 

 sequently 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 shell surface. In the accompanying figure, the speci- 

 mens from the Conway estuary and the Solent( 12,5) well il- 

 lustrate this latter form of shell, while that from exposed rocks 

 is illustrated by the specimens from Robin Hood Bay (13, 14.)." 



On pages 69-70 in the same article Cooke refers to the possi- 

 bility that a selective factor may be involved in determining 

 the coloration of the shell. He indicates this in the following 

 language:- " The specimens oi Purpura lapillus which occur 

 in Cornwall are banded with rings of color, especially black 

 and white, in a more striking way than any other specimens 

 that have occurred to my notice. I am inclined to refer this 

 peculiarity to a tendency toward protective coloration, since 

 the rocks on which the Purpura occur are often banded with 

 white and color, and variegated to a very marked extent.'* 



Dr. W. H. Dall (1915) in his paper dealing with the forms of 

 Thais belonging to the subgenus Nucella found on our west 

 coast has expressed views as to the origin of the local races on 

 purely environmental grounds as follows:- " The sheltered 

 rocky beaches of a well protected harbor will afford slender 

 elongated and lamellose specimens with small apertures. The 



