STATION IN FRESH-WATER MUSSELS. 311 



large rivers, and passes gradually, in the upstream direction, into a 

 less obese (compressed) form in the Jieadzvaters; 



2. zuith the decrease in obesity often an increase in size {length) 

 is correlated; 



J. a feiu shells zvhich have, in the larger rivers, a pecidiar sculp- 

 ture of large tubercles, lose these tubercles in the Jieadzvaters. 



The question arises : zvhat is the meaning of these changes in 

 shape? No positive conclusion is as yet possible, chiefly for two 

 reasons: first, that there are only some species (and comparatively 

 few^), in which this law is observed, while others positively do not 

 show it ; and in the second place, that, although the size of the stream 

 undoubtedly is connected with this phenomenon, we do not know, 

 whether size alone is the essential factor, or whether additional 

 factors belonging to those constituting the small-stream-community 

 are responsible. 



A few points, however, should be mentioned, which might finally 

 lead to or help in the proper understanding of the facts. 



1. Practically all of the shells which show this phenomenon are 

 of a rather primitive structure. The genera Fusconaia, Amblema, 

 Quadrula, Lexingtonia, and Pleurobema, belong to the most primi- 

 tive types of North American Naiades ; and Dromus and Obovaria 

 are comparatively primitive among the subfamily Lampsilincr. No 

 Naiades which stand on a high stage of dififerentiation have given 

 any distinct evidence for our law. 



2. It must not be forgotten that dispersal of almost all our 

 Naiades is accomplished in the larval stage, when the larvse live 

 parasitic upon fishes, and that certain species of shells are re- 

 stricted to certain species of fishes as hosts. Thus the distribution 

 of the fish-host, and the ecological peculiarities of it, must largely 

 influence the distribution of the Naiades. Since we have fishes 

 which are migratory, while others are more stationary, it might be 

 that Naiades living parasitic upon the latter kind have a smaller 

 chance to be carried far away from their native grounds, while 

 others, parasitic upon migratory fishes, are promiscuously scattered 

 over the whole river-system, being often deposited far from their 

 place of birth. In the first case, development of local races, in 



