LAND AND FRESHWATER MOLLUSCA OF MANITOBA. 34 1 



Of the 72 species which I am able to record, only i6 inhabit 

 the land, and even they are only found in moist situations 

 where the fire seldom penetrates. This great dearth of terres- 

 trial species might, on first thoughts, very naturally be attributed 

 to the excessive frosts, but I believe I am able to assign to it a 

 much more direct cause, viz.. Fire. If it were attributable to 

 cold, how is it that thousands of individuals are able to exist in 

 shallow ponds that must certainly become frozen solid during 

 winter ? I have elsewhere entered fully into the subject of the 

 effect produced on the face of the country by the extensive 

 prairie-fires that have annually swept over it for generations 

 past* There are good reasons for believing that the very 

 prairies themselves, their treelessness, and their fertiliy are 

 all due, to a large extent, if not entirely, to the action of these 

 fires. I have further stated my belieff that the complete 

 absence of earth-worms of every kind from the surface of the 

 prairies is, in all probability, due to the same cause ; and I see 

 no reason to suppose that the remarkable absence of land shells 

 is due to any other. The fire annually burns the grass over 

 which it passes, completely down to the ground, and I have had 

 many occasions of observing that this would effectually kill any 

 mollusks that were harbouring among its roots. On the drier 

 portions of the prairies, settlers often cut their hay round the 

 margins of small depressions in which water collects, and shells 

 — especially Limnaeidae — live during the spring. This done, 

 they set fire to the remaining grass-stalks in order, as they say, 

 that the grass may come up greener and more succulent the 

 following spring. Under such conditions I have often seen 

 the shells lying on the dry pond-bottom completely scorched 

 and calcined by the flames. It seems to me, therefore, in every 

 way probable that these prairie fires are the cause of this absence 

 of terrestrial mollusks from the face of the country, especially 



* 'Manitoba Described,' p. 20. Wyman & Sons, Great Queen Street, 

 W.C. 1885. 



t ' Nature,' Jan. 3, 1884, p. 213. 



