44 



The vertical lamellae are likewise subject to wide variation. 

 In many populations these are completely obsolete or feebly 

 expressed, while in others they entirely dominate the sculp- 

 tural pattern, resulting in such highly imbricated forms as 

 those shown in Plate L. Intergrading with these extremes we 

 find every imaginable variation, not only as seen in separate 

 populations but also within the limits of a single population, 

 as may be seen in the specimens shown in Plates xxixand 

 XL., where a clinal system is developed. 



The form of the spire, which largely determines the general 

 contour of the shell, is also regarded as determined by genetic 

 factors. The degree of acuity varies greatly, ranging from cases 

 where the width of the shell is fully or nearly as great as its 

 height, to forms in which the height is several times greater 

 than the width, as appears in some of the Alaskan forms 

 illustrated in Plates liv, Lv. Even within the same population 

 there is often considerable variation in proportions. 



Size is also determined primarily by heredity, but is influ- 

 enced to an important extent by nutritive conditions in the 

 habitat. The snails as adults feed mainly, if not exclusively, 

 upon barnacles and mussels. Where these are abundant and 

 the snails live upon submerged reefs, in protected coves or on 

 the piling of docks, the growth rate is stimulated, and large or 

 even "giant" forms are produced. The presumed differential 

 effect of environmental conditions may be illustrated by two 

 populations occupying contiguous areas with extremely differ- 

 ent ecological conditions. One population (Plate xxiv) occurs 

 in a protected cove on Blown Island in Friday Harbor, where 

 average ecological conditions are present, and here we find 

 individuals of average size. Across a narrow channel a short 

 distance away there is at Point Caution a rocky, wave swept, 

 rather barren area inhabited by a population (Plate xxv), 

 which although of the same physical type is seemingly dwarfed. 

 On the other hand, populations inhabiting submerged reefs, 

 where food is abundant and the feeding process is not inter- 

 fered with by storms or tidal changes, growth is speeded up 



