STATION IN FRESH-WATER MUSSELS. 311 
large rivers, and passes gradually, in the upstream direction, into a 
less obese (compressed) form in the headwaters; 
2. with the decrease in obesity often an increase in sige (length) 
is correlated; 
3. a few shells which have, in the large rivers, a peculiar sculp- 
ture of large tubercles, lose these tubercles in the headwaters. 
The question arises: what 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 sige 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 Pusconaia, 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 Lampsiline. No 
Naiads which stand on a high stage of differentiation have given 
any distinct evidence for our law. 
2. It must not be forgotten that dispersal of almost all our 
Naiads is accomplished in the larval stage, when the larve 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 
