230 



BRACKISH- WATER MOLLUSKS. 



An acquaintance with these intermediate areas, 

 and the zoological features they present, is indis- 

 pensable to the physical geologist. Throughout 

 the long series of old secondary and tertiary for- 

 mations, like conditions are constantly presented. 



Apart from minor areas of brackish water the 

 North Atlantic passes, in the ultimate portions 

 of two of its great lateral branches, into internal 

 seas, which differ from estuaries only in their extent. 

 They communicate with the ocean, and are brack- 

 ish from the fresh waters poured into them. 



The brackish-water fauna of our European area 

 varies much according to the province to which it 

 belongs. In our Celtic region Rissoa, Assiminia, 

 Neretina, Gonovulus, and Truncatella, habitually 

 prefer the mouths of estuarine rivers, and low, sea- 

 side pools. Littorina littorea ranges away upwards 

 from the pure sea water, but seems to suffer from 

 the change. Limnceus pereger ventures downwards. 

 Scrobicularia and Mactra solida may be taken as 

 good characteristic estuary shells ; they are at their 

 maximum in such places, and attain their largest 

 dimensions. Cardium edule is common, as is Mya 

 arenaica, but both dwarfed. 



" When visiting," says Ed. Forbes, " the great 

 South Arran, in company with Mr. Thompson, we 

 found an interesting variety of the Cardium edule, 

 in a brackish lake, at the northern end of the 

 island. The shells were remarkably thin and 



