192 LAND AND FRESH-WATER MOLLUSKS. 
summer’s day traversing the surface of the water 
in an easy undulating line, or floating in luxurious 
repose, perhaps 
To taste the freshness of heaven’s breath, and feel 
That light is pleasant, and the sunbeam warm. 
When about to descend from the surface, it con- 
tracts the pulmonary pouch, expelling the air, 
and drops at once to the bottom, and regains the 
surface by crawling up some solid body. 
This species 1s distributed throughout HEng- 
land, but is rare in the north; it inhabits Gulane 
Loch, 17 or 18 miles east of Hdinburgh; it is 
rare in Ireland. The variety is found in a few 
canals. I have met with it very elongated in 
the River Lea, at St. Alban’s. 
I. stagnalis furnishes us with one of the pro- 
blems of Natural History —its disappearance 
in localities where it has been more or less 
abundant for some years. Thus Dr. Johnston 
recorded the presence, and after a short lapse 
of time the total disappearance, of this snail in 
Berwickshire. Having observed this phenomenon 
in part, I am enabled to give the following pro- 
bable explanation :—A pond in the neighbour- — 
hood of Hastings was tenanted by many adults 
of LD. stagnalis ; in the following year they were 
not to be found; but in the summer of the suc- 
ceeding year I found them again in plenty. 
