PRESENT HISTORY AND DISTRIBUTION 59 



and Pisidium, Plate XV., Fig. 6) climb about in the 

 weed. Freshwater Mollusca are also not infrequently 

 found living in water-pipes and cisterns connected 

 with artificial supply. 



Altitude has no terrors for the freshwater Mol- 

 lusca, Limncea Hookevi having been taken at a height 

 of 1 8,000 feet above the sea. On the other hand, 

 at a depth of 180 fathoms in Lake Baikal, some 

 Paludestrinidae peculiar to those waters have been 

 met with. The most northern occurrence of a 

 freshwater Snail is that of Aplecta hypnovum (a 

 British species) which in Siberia attains as far north 

 as 73° 3°' m the Taimyr Peninsula. 



Just as the marine Mollusca break their bounds, 

 so will the freshwater pass theirs, and essay to 

 trespass on the land. The Limnseidae commonly 

 crawl on the wet banks out of the water and in 

 moist spots where swamp-loving terrestrial Pul- 

 monates, like the Amber-Snails (Succinea, Plate XIII., 

 Fig. 14), etc., love to pass their existence. 



Among marine forms found on land may be named 

 a remarkable Periwinkle, Cremnoconchus, which occurs 

 only on wet cliffs in the Ghats of Southern India, 

 thirty to fifty miles from the sea. Another Peri- 

 winkle, Littorina arboricola, lives on trees fully 100 

 yards from the backwaters of Trincomalee Harbour, 

 and is never known to enter or be covered by the 

 water. The Rhipodoglossate, Neritodryas, a relative 

 of Theodoxis, occurs on trees of some height in the 



