148 Annals of the Carnegie Museum. 



form as broad or broader than long from sandy bottoms; (c) a thick- 

 shelled form from rapid waters. He adds that forms from still water 

 and muddy streams have a prolonged posterior end, which is not 

 so well developed in rapid currents. 



M. C. March (30) apparently began a statistical study along the 

 same lines pursued in this paper, plotting variation curves based 

 upon the relation between the antero-posterior or horizontal axis and 

 the dorso-ventral or perpendicular axis. Only a short note was pub- 

 lished, of which the following summary is given. March states that in 

 Unto tumidus and Unio pictorum two main types of shell occur: one 

 stout and heavy with relatively long dorso-ventral axis; the other 

 with short antero-posterior axis, etc. March believes that the growth 

 of the anterior portion of the shell is slower than that of the posterior 

 portion, "as is natural with an animal, which has to plough its way 

 through the mud." An increase in the rate of the current in which 

 they live would produce a decrease in pre-umbonal development, and 

 thus tend to give the umbo a more forward position in those forms 

 which inhabit strong currents. Such forms are found in canals having 

 strong gradients, those otherwise modified in deeper canals with an 

 abundance of locks. Thick mud is supposed to induce elongation, 

 slow rivers develop forms with long dorso-ventral axes. 



A paper by Clessin (6) published later contains many interesting 

 side-lights upon the history of the Naiades, but nothing of immediate 

 interest to us. Finally Haas & Schwarz (15) propose as a law the 

 statement, which this paper endeavors as the result of investigation 

 to prove, that "The same types under the same biological (ecological) 

 conditions produce the same variants; different types under like con- 

 ditions produce convergent (parallel) local variants. In the case of a 

 sufficiently lengthy isolation the local variants subject to biologically 

 similar environments, may become constant or fixed local forms." 

 {Free translation.) 



The above completes a resume of all the European literature to 

 which I have had access. While a large part of it, as well as the litera- 

 ture hereafter cited, may not appear to have much bearing upon the 

 results of the following investigation, I believe that a reference to 

 it is essential to a complete understanding of the nature of the problem. 

 The following references to investigations made upon our i\merican 

 forms are added as bearing upon the same or closely related species, 

 with which I deal later. 



