PRESENT HISTORY AND DISTRIBUTION 53 



Experiments conducted many years ago show that 

 species of the pulmonate genera — Limncea, Physa, 

 Planorbis, and Ancylus — can be habituated by the 

 gradual addition of salt to as much as 4 per cent, 

 in the water ; so, too, but less easily, can the Proso- 

 branchs Vivipara, Bithynia, and Theodoxis ; while the 

 Pelecypods Anodonta, Unio, and Sphczrium die before 

 that degree of salinity can be attained. Conversely, 

 marine forms can be gradually accustomed to fresh- 

 water existence : the Mussel (Mytilus) very easily ; 

 the Cockle (Cavdium edule), the Oyster, the Common 

 Limpet (Patella vulgata), Turbo neritoides, and others, 

 less successfully. In these experimental cases, how- 

 ever, propagation would not take place. 



On the rocks by the margin of the sea, within 

 reach only of the splash of the waves, or of the 

 water at the highest tides, will be found certain of 

 the Periwinkles (Littorinidse) — a situation in which 

 they are joined by such of the Pulmonates as several 

 of the Auriculidse and the slug-like Oncidium, that 

 dwell close down by the sea margin. 



A little lower down, just below high-water mark, 

 those strange Limpet-like Pulmonates, Siphonaria, 

 Gadinia (Plate XXVI., Figs. 24 and 25), and the 

 recently discovered Aporemodon, that have partially 

 reverted to marine life, are to be found. 



Between tide-marks the Chitons, Limpets (Patel- 

 lidse), Keyhole Limpets (Fisurellidae), Ormers (Halio- 

 tidse), and similar molluscs, cling to the rocks or the 



