Qiiarh'iiy Journal of Coiicliotogy. 339 



ON CERTAIN SPECIES OF LITTORINA. 



By J. S. Gibbons, M.B. 



In tropical aiid subtropical regions certain species of Lif- 

 toriiia are confined to water more or less brackish, being incapable 

 of living in pure salt water. 



I have met vrith three of these and in each case they have been 

 distinguished from tlie truly marine species by the extreme (com- 

 parative) thinness of their shells and by their coloring being richer 

 and more varied ; they are also usually more elaborately marked. 

 They are to be met with under three different conditions, viz., in 

 harbours and bays where the water is salt v/ith but a slight admix- 

 ture of fresh water : in mangrove sv/amps where salt and fresh 

 vrater mix in pretty equal volume; and lastly on dry land, but 

 near a marsh or the dry bed of one. 



L. infen/iedin, Rve., a widely-diffused E. African shell, attaches 

 itself by a thin pellicle of dried mucus to grass growing by the 

 margin of slightly brackish marshes near the coast, resembling in 

 its mode of suspension the Old 'World Cjdosiofnas. I have found 

 it in vast numbers in situations where, during the greater part of 

 the year, it is exposed to the full glare of an almost vertical sun, 

 its only source of nioisture being a slight dew at night time. The 

 W. Indian Z. angulifera, Link, and a beautifully colored E. African 

 species (Z. carinifera. Juke?) are found in mangrove swamps, &c., 

 tliey are however less independent of salt wg^ter than the last. 



Notwithstanding that the true marine species are tliickcr than 

 those found in Ijrackibh water, the latter become more solid as 

 the water they inhabit becomes less salt. This is curious and the 

 reverse of what one w^ould expect. It is, however, undoubtedly 

 the (.a.,e, as I have often satisfied myself. Z. afigulifera, e.g., is 

 unusually solid and heavy at Puerto Plata (S. Domingo) among 



