18S4.] NATURAL SCIENCES OF PHILADELPHIA. 211 



stant improvement in the food-supply or in the siliceous constit- 

 uent of the water. He has traced the workings of the rule more 

 particularly through the very variable species, Spongilla lacustris 

 and S.fragilis; in Meyenia fluviatilis^ in Heteromeyenia argyro- 

 sperma and H. Kyderi, and lastly and most conspicuously in 

 Tuhella Fenvsylvanica. The extremes in this last series differ so 

 widely that they wJbld hardly be taken to belong to the same 

 species, but the intermediate grades have all been collected, largely 

 from the same stream ; and as a result several species named in this 

 and other cases, have relapsed into synonyms. 



September 9. 



Dr. W. S. W. RuscHENBEEGER, in the chair. 



Eleven persons present. 



The death of R. E. Rogers, M. D., a member, was announced. 



September 16. 

 Rev. H. C. McCooK, D. D., Vice-President, in the chair. 

 Seventeen persons present. 



On the Minute Fauna of Fairmount Beservoir. — Mr. E. Potts 

 alluded to the difficulties that ordinarily prevent a thorough study 

 of the fixed aquatic fiiuna, which he described as thereby generally 

 limited to collections from the shallow water near the margins of 

 lakes and streams, or of such forms as may adhere to the few 

 timbers or stones that can be dragged from a greater depth. He 

 therefore urged the importance of making use of such opportuni- 

 ties as are furnished by the temporary drainage of reservoirs, 

 canals, etc., to examine thoroughly the incrustations upon exposed 

 walls and timbers, or on the bed of the stream. 



Such an occasion was affoixled a few days since, when the acci- 

 dental breaking of a valve necessitated the drawing ofi' of the 

 water from tlie Fairmount reservoirs. These are divided by per- 

 pendicular walls, eight or ten feet in height, and, unfortunately, 

 facilities were not at hand in the shape of ladders, planks, etc., to 

 enable him to make a minute examination of them. From the 

 margin, however, could be seen at many places patches of the 

 s\do\)ges, Spongilla fragilis and Meyenia Jiuviatilis^whila the cages 

 over the outlet ])ipes, and, more strikingly, the walls surrounding 

 the main outlet at the southeast corner, were thickly encrusted with 

 Meyenia Leidyi. The last-named sponge is very compact and 

 little liable to crumble during the winter season, so that it is prob- 

 able that the large masses, some of them nearly an inch in thick- 



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