SOME LITTORAL TROCHID^ AND LITTORINID.^ IN CARDIGAN BAY. 119 



Both were small, flattened, and occurred beneath stones. L. ohtusata 

 accompanies Fucus, and is the most persistent species throughout. 

 L. littorea appears about the same time as G. umbilicata, but does not 

 become abundant until fresh-water influence is an appreciable factor ; 

 it then increases in number and size. G. umbilicata is dominant for a 

 considerable distance, but as the river is approached, M. crassa appears, 

 here one and there one, amidst the crowds of Gibhula. Those first noticed 

 are rather small, but there is a gradual increase in size and abundance 

 until it equals G. umbilicata in numbers. That species then dies out as 

 fresh water is approached, and is replaced by a rather squat, solid, 

 smooth form of L. rudis, which in its turn rapidly becomes larger and 

 more numerous, while M. crassa, after being a dominant species for <" 

 while, dies out in its turn. Then for a space, L. rudis and L. littorea alone 

 occupy the ground ; but both are absent from the actual flow of the 

 stream. On the northern side of the river, mussels are abundant in the 

 crevices of the stones for some distance along the shore, and are accom- 

 panied by L. rudis and L. littorea. Then, as the mussels disappear, 

 M. crassa reappears, and in company with the Littorinas becomes very 

 abundant. G. umbilicata recommences by degrees, increases in numbers, 

 and with M. crassa continues until the boulder area once more gives 

 place to shingle. 



This lateral distribution or zoning may be expressed thus (the order 

 in each instance is that of abundance) : — 



