94 ORTMANN— THE DESTRUCTION OF [April 23. 



destroys them. They seem to be sHghtly more resistant than the 

 Unionidse (see below), but their presence in a polluted stream is 

 in many cases clearly due to a restocking of the stream, by immigra- 

 tion from a clear tributary. The crawfishes are rather vagile, and 

 possess the power to migrate, although less so than the fishes. There 

 surely is the possibility for them to take advantage of a temporary 

 improvement of the condition of a stream. 



The most important group, with reference to the matter in ques- 

 tion, are the bivalve mollusks of the family Unionidse, the fresh- 

 water mussels or river-clams. They are the most reliable indicators 

 of the pollution of a stream. Being rather sedentary, living on the 

 bottom of the rivers, breathing water, they are easily influenced by 

 the deterioration of the water. Of all the more important groups 

 of our fresh-water fauna, they die first, and after they have been 

 exterminated, it is exceedingly difficult to restock the stream on 

 account of the complex life history of the young mussels. It is 

 known that the young Unionidse are transported and dispersed by 

 fishes, but in a polluted stream the fishes have also disappeared, and 

 even in a case of a temporary recovery of a stream, in times of a 

 high stage of the water, if there should be a restocking with young 

 mussel-fry, the latter will surely be killed during the next low stage, 

 when the pollution again is concentrated. In this respect the Union- 

 idae surely are worse off than the fishes and crawfishes. 



Of other mollusks, the gasteropods belonging to the family Pleu- 

 roceridae (Plenrocera, Goniobasis, Anculosa) should be mentioned. 

 They are generally absent in polluted rivers, but have been found 

 surviving, together with crawfishes, in parts where Unionidse were 

 entirely, and the fishes for the greater part gone (Allegheny River 

 in southern Venango County). Other mollusks, which are air 

 breathing (genera Lynmcsa, Planorhis, Physa) are more resistant, 

 and this is especially true of Physa, which represents in certain 

 instances the only remaining life in certain rivers. But there also 

 seems to be a limit to its power of endurance, and in very badly pol- 

 luted streams also Physa is absent. 



Thus we can establish, in a rough way, a certain succession for 

 the disappearance of our fauna. 



The first sign of pollution of a dangerous character in a stream 



