310 ORTMANN—CORRELATION OF SHAPE AND 
be due, by any means, to accident. Thus we must accept it as 
demonstrated, that in most shells which show a decrease in diameter 
in the upstream direction, this decrease is compensated, to a degree, 
by the greater size of the shell as expressed by its length. 
Further, we have seen in certain forms (Quadrula metanevra 
and Qu. cylindrica, Dromus dromas), that a peculiar kind of sur- 
face sculpture, namely large knobs and tubercles, tend to disappear 
in the headwaters. This is also connected, more or less distinctly, 
with a flattening of the shell. It would be interesting to discover 
additional cases of this kind, and may be found in Truncilla torulosa 
and gubernaculum of the upper Tennessee. But it surely is not a 
general law, since there are other shells, which show no change in 
sculpture along the course of a river, and since there are other in- 
stances, where the opposite seems to be the case. The group of 
Amblema plicata seems to belong here, where a strongly sculptured 
and flat form (A. costata) belongs to the headwaters, while a 
smoother and more swollen form (peruviana) is in the largest 
Trivers. 
In this connection it should be pointed out, that a similar case is 
known in the freshwater Gastropod Jo. The facts have been posi- 
tively established by C. C. Adams**; a tuberculated or spinose form 
(Io spinosa Lea) is found in the larger streams; a smooth form (Jo 
fluvialis Say) in the headwaters. The present writer is able to con- 
firm this by his own observations. The smoothness of the speci- 
mens of Jo in the upper Powell, Clinch, and Holston in Virginia is 
very striking ; tuberculate individuals begin to appear farther down, 
and real spiny ones not till the state line of Tennessee is reached. 
The transition is quite gradual. 
CONCLUSIONS. 
Certain Naiades change their shape along the course of one and 
the same river in such a way, that 
I. the more obese (swollen) form is found farther down in the 
22 See Wilson and Clark, ’14, p. 63; Utterback, 716, p. 41. 
23 Mem. Nat. Acad. Sci., 12, 1915. 
