Mollusca of Lower Canada, 453 



of heat and cold, together with the dryness of the atmosphere in 

 Canada seem unfavourable to the abundance of land snails. Hence- 

 we must not expect, perhaps, to find many novelties among the 

 terrestrial mollusca, except among the small and critical species. 

 But in this land of lakes and mighty rivers, which may almost be 

 said to be unexplored, many interesting fresh water shells may 

 yet be obtained. 



Of the Unionida3, four species new to the published lists, have 

 occurred to me in Lower Canada. Three of these are New Enor- 

 land species, while the other was described from the Ohio river.. 



In the rivers, lakes and swamps, throughout the whole pro- 

 vince, living in the sand or mud at the bottom, there occur 

 small bivalves of the genera Cyclas and Pisidium. The chief 

 difference between Cyclas and Pisidium is that in Cyclas the two- 

 siphons are distinct, while in Pisidium the siphons are united 

 into a single tube. The shell of Cyclas is nearly equilateral, 

 while that of Pisidium is very oblique. These creatures are most 

 abundant everywhere, but, comparatively speaking, very little is 

 known respecting them. I have eight species not previously re- 

 corded as Canadian, while in the proceedings of the Boston Natural 

 History Society, ten species new to science are recorded from the 

 neighbourhood of Lake Superior. I would call special attention 

 to these little shells ; the fact of no less than eighteen species having- 

 been left out in all the catalogues of land and fresh water shells 

 in the Canadian Naturalist, would seem to shew that our rivers^ 

 and lakes may contain many rare and curious forms which have 

 yet to be detected. 



The remaining three species are Limnsea columella, Planor- 

 bis armigerus and P. deflectus; three fresh water snails, mostly 

 critical forms, .which have been previously overlooked. A most 

 remarkable fact in connection with these fresh water snails,, 

 is that no less than nine species, a large proportion of the whole, 

 occur on both the Atlantic and Pacific coasts. It has been 

 held by many naturalists, that a lofty mountain chain will 

 form an obstacle to the migration of species. Yet here we 

 find that on each side of a mountain barrier, some of the 

 peaks of which are as much as 15,000 and 16,000 feet above the 

 level of the sea, and clothed with perpetual snow, such sluggisli 

 creatures as fresh water mollusca both can and do exist, the species 

 in each case being identical. It would seem at any rate, that 

 there are exceptions to this rule, and that the Rocky Mountains^ 

 'or example, do not present an insuperable obstacle to migration.. 



