116 CHAS. L. WALTON. 



mark, and their stratified wall-like sides afford crevices, inhabited by 

 this species and L. rudis. L. neritoides, however, only occurs on the 

 upper portions, and on the N. and E. faces. A few may be observed 

 on the S.E. also, but this is exceptional. I have seen practically the 

 same thing in similar positions on the Devon coast. On the sea walls of 

 the promenade at Aberystwj^th, this species occurs, chiefly where the 

 rays of the summer sun will not strike about midday. 



(8) L. obtusata occurs throughout the littoral region examined wherever 

 Fucus grows, and hence is the most thoroughly persistent form. 



(9) L. rudis. — This also is generally present, and frequently exceed- 

 ingly abundant. It usually accompanies L. littorea in its distribution, 

 though at a somewhat different level. These two frequently appear and 

 disappear simultaneously, but one or other may be locally dominant. 



Vertical DiSTRiBrTioN. 

 The distribution of the species may also be considered vertically, that 

 is, in zones. These zones, of course, overlap considerably and are seldom 

 all well developed, or even present in any given locality. By contrasting 

 varying localities a general succession appears. This is not by any means 

 arbitrary, as some of these zones may occur intermixed at times, particu- 

 larly in the case of M. crassa, L. littorea, and L. obtusata. L. yieritoides is 

 constant when conditions permit, and L. obtusata is dependent on the 

 presence of Fucoid Algae. C. zizyphinus has occurred once, but in its 

 normal position. 



Allowing for local differentiation and overlap, the succession is, more 

 or less, as follows :■ — 



L. neritoides. 

 L. rudis. 

 (M. crassa. 

 JL. obtusata. 

 yL. littorea. 



G. umhilicata. G. magus. 

 G. cineraria. 

 C. zizyphinus. 



The succession of the Littorinidse can be best followed in the Aberyst- 

 wyth district, where the Trochidae are absent. 



On the sea wall of the promenade below the University the following 

 succession can be observed : — 



(a) Barren stonework. 



(6) Zone of L. neritoides. 



