27 



In some cases the major spirals, especially where these arc 

 strongly developed .are spaced by delicate thread-like second- 

 ary spirals, which are very suggestive of the corresponding 

 structures in lapillus. 



The axial sculpture in lamellosa exhibits great extremes. 

 The ridges may be entirely absent, presenting us with a com- 

 pletely smooth shell, or the ridges may be raised in conspicu- 

 ous lamellae, giving rise to the imbricate type of sculpture, 

 with all possible intermediate gradations in ornamentation'. 

 The sculptural pattern of the shell in its turn is modified by 

 several factors. The general type of the shell, whether long 

 spired or short spired, tends to determine whether the lamellae 

 will be crowded or widely spaced. The size of the adult shell 

 appears to be determined by the basic heredity of the popula- 

 tion and the speed of growth in reaching maturity, and in 

 turn is related to the abundance and availability of the food 

 supply. Snails living in the intertidal zone are more or less 

 intermittent in their feeding operations, while those living on 

 submerged reefs where as a rule larger types of barnacles are 

 abundant, ic^d without interruption, and hence grow more 

 rapidly and mature to a greater size. The presence of "giant" 

 forms of this type has been noted for lapillus in the Atlantic, 

 and is frequently seen in populations of lamellosa. In such 

 cases the axial ridges may be widely spaced. The size of indi- 

 viduals in populations inhabiting different types of shore in 

 the intertidal zone exhibit the same kind of variance. It has 

 also been noted that colonies of lamellosa established upon 

 the barnacle encrusted piles of docks usually attain a greater 

 size than the populations inhabiting the adjacent shores, from 

 which presumably they have been derived. 



In dredging over bottoms that lie in deeper water and are 

 covered with loose shelly debris, Thais material is frequently 

 brought up containing specimens that are of moderate size; it 

 IS assumed that the conditions in such areas are unfavorable 

 for the animals, thus retarding their growth. 



