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Stri pmiiatid jhiinti varies witli it. Vov iiislaiici', in si>iiic' piact's wlicre iiiiiildy iiot- 

 touhs, and an aitiindant Mora co-exist, tlic several nienihers ol' tlie jjenus Pleurocera 

 ahonnd. At another, where the l)ottoni is (dean rock, or is rock with ahnnthint 

 confervoid vegetation, the ^^vwws Ancnlom ix'iiirs in tlie i,M-e;ilest pnifnsion. At the 

 niiiuerons small falls over the rocky Hats, where the water is indifferently swift, 

 an<I the bottom is either clean or with scanty veijetation, are fonnd threat numbers 

 of the four species of (ioniohaxis. At another time in tlie year, when the stage of 

 the water is changed, a rather difi'erent distrihntion, locally, may he noted. These 

 relations exhibit a certain dejiendence on local conditions that vary, and, per- 

 liaps, serves to explain tlie very ditrerenl character of the sludi fauna at the same 

 place, at different periods of the year. 



The eiirliest forms that have been described from tlie Falls are now unknown. 

 They were discinered and studied i)y the nnfortuinite Professor Ratines(]ne, and 

 have long since been merged into synonymy by other students, wdio were unwill- 

 ing to allow his claim to original discovery. The attempt lately has been made, 

 with indifferent success, to fix these forms. What result more extensive study of 

 the literature of conchology will tinally justify must be left for another time and 

 placi'. Here it is simply the purpose to placi' on record the forms which occur, 

 their synonymy as now understoitd, and a study of those changes in form and 

 habit wliicli m;iuifcstly result from the environment of the various representa- 

 tives of the family. 



The specit'S of P/eiinicera are the following: PlKitrocera canxiliculdtiim. P. 

 moniUferinu and P. elerafnm. There are many specimens which are so difhcult of 

 determination, when studied in large series, that one is inclined to the view that 

 forms of extreme variation. l)nt really specifically related, have been given species 

 names which ought to have i)een not even recognized as varieties. A si)ecies 

 monger could erect, by carefully selecting his examples, a dozen or more species 

 from the simple variations in coloration alone, and, strange to relate, this has been 

 <lone. Thus Plenrocera runalirulalmii has occurred in abuiidance with one, two, 

 three and even four revolving jjurple bands. Many s])ecimens have been secured 

 which are entirely pnr{)le, and with no send)lance of distinctness in the banding. 

 Hundreds of individuals have lieen taken that are iu'ight, honey-yellow, and have 

 no tendency to other coloration whatever. Many present the character of chan- 

 neled whorls, on which the specific name is based, while as many more are found 

 that have plain and w>dl rounded whorls, without any indication of the so-called 

 characteristic grooving. The form calletl P. I'lrrutuin itself is a beautiful illustra- 

 tion of the effects of <lifterent environment. If taken from swiftlv tlowinj' water. 



