Ill VARIATION IN LITTORINA 93 



water, and Llyn-y-van-fach. It lives adhering to stones in places 

 where there are very few weeds, its shape enabling it to with- 

 stand the sm-f of these large lakes, to which the ordinary form 

 would probably succumb.^ 



Scalariform specimens of Planorhis are said to occur most 

 commonly in waters which are choked by vegetation, and it has 

 been shown that this form of shell is able to make its way 

 through masses of dense weed much more readily than specimens 

 of normal shape. 



Continental authorities have long considered Limnaea percgra 

 and L. ovata as two distinct species. Hazay, however, has 

 succeeded in rearing specimens of so-called fcregra from the ova 

 of ovata, and so-called ovata from the ova of jperegra, simply by 

 placing one species in running water, and the other in still 

 water. 



According to Mr. J. S. Gibbons ^ certain species of Littorina, 

 in tropical and subtropical regions, are confined to water more 

 or less brackish, being incapable of living in pm-e salt water. 

 " I have met," says Mr. Gibbons, " with three of these species, 

 and in each case they have been distinguished from the truly 

 marine species by the extreme (comparative) thinness of their 

 shells, and by their colouring being richer and more varied ; 

 they are also usually more elaborately marked. They are to be 

 met with under three different conditions — (1) in harbours and 

 bays where the water is salt with but a slight admixtiu-e of fresh 

 water; (2) in mangrove swamps where salt and fresh water mix 

 in pretty equal volume ; (3) on dry land, but near a marsh or 

 the dry bed of one. 



" L. intermedia Eeeve, 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, re- 

 sembling in its mode of suspension the Old World Cyclostoma. 

 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 som-ce of moistm-e being a slight 

 dew at night-time. The W. Indian L. angulifera Lam., and a 

 beautifully coloured E. African species (J L. carinifera), are found 

 in mangrove swamps ; they are, however, less independent of salt 

 water than the last." 



^ J. Madisou, Journ. of Conch, v. p. 260. - Quart. Journ. Conch, i. 339. 



