PRESENT HISTORY AND DISTRIBUTION 53 
Experiments conducted many years ago show that 
species of the pulmonate genera—Limnaa, 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 Spharium 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 (Cardium 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 (Littorinide)—a situation in which 
they are joined by such of the Pulmonates as several 
of the Auriculidz 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 Apovemodon, that have partially 
reverted to marine life, are to be found. 
Between tide-marks the Chitons, Limpets (Patel- 
lide), Keyhole Limpets (Fisurellidz), Ormers (Halio- 
tidz), and similar molluscs, cling to the rocks or the 
