Fresh-Water Shells. 195 



progi-ess of the struggle, they were in turn displaced. The 

 changes in the occupancy of a given area in the sea bottom, must 

 often have been of this character ; and where the facts can be as- 

 certained, they form good illustrations of the slow and somet-'mes 

 interrupted manner in which new faunas, sj)ri?ading from their 

 centres of creation, have extended themselves ovor the earth. 



M. Barrande regards the oldest or '■'• Primordial " fauna of 

 Bohemia as equivalent to the English " Cambrian" and to our oldest 

 " Silurian " beds in America. Murchison, we rather think, will 

 claim them as the lower members of his kingdom of " Siluria."* 



.7, w. D. 



ARTICLE XXII. — Descrqjtioa q/ sorne of the Fresh-'water Gaster- 

 opoda, inliabithig the Lairs and Rivers of Canada. 



In the following article we have transcribed from several works 

 descriptions of nearly all the moUusoa of the family Limn^ad^ 

 that are to be found in the fresh wateis of Canada. Their shells 

 are more or less abundant along the shores of all the lakes, ponds, 

 or rivers of the country, and also constitute those valuable depo- 

 sites knowai as shell marl. One of these beds of marl may be 

 seen in the suburbs of the City of Montreal, where it has been 

 laid open in the ditches crossing the Lachine Railway. Seven or 

 eight of the species hereinafter described may be procured at that 

 locality. In the ponds at the quarries east of the city, some of 

 the LimneiB and Physfe are also plentiful. "We have not seen the 

 large species, L. StagnaUs, in this vicinity ; but near the City of 

 Ottawa, it is common in the Rideau river and canal. The figures 

 given below are copied from an English work, but they represent 

 our species very nearly. A few days since we showed some of 

 the Canadian specimens to a naturalist from Britain, then on a 

 visit here, and he said they were scarcely to be distinguished from 

 those common in the ponds and ditches in England. Ours is not 

 quite so much angulated upon the upper part of the whorl. 



* Since the above was written wc have found that a second edition of 

 the supplement has appeared, containing other new facts among whicli 

 is the discovery of mammals in the secondary rocks of America. 



