Grier: Morphological Features of Mussel-shells. 177 



Erie than those of the headwaters, shells of the same specific stock 

 have the posterior part better developed than forms from the tribu- 

 taries. Conditions are more regular in Lake Erie, but the impulse 

 to migrate less frequent, and if there be such a thing as adaptation, 

 enlargement of foot correlated with enlargement of shell apparently 

 follows in the Upper Ohio. To the two exceptions, Eurynia and L. 

 ovata, the following remarks may apply. The former appears to enjoy 

 coarse or fine gravel in a strong current in large and moderate rivers, 

 never small streams, and often burrows deeply. Under these condi- 

 tions, if the hypothesis of modification of shell holds true, we should 

 expect to find a greater development of the anterior portion, which 

 we do not. Apparently, the only consistent explanation is to suppose 

 it requires a smaller foot for burrowing than in moving against the 

 current in the Upper Ohio. Possibly the compression of shell may ac- 

 count to some extent for this. In this connection it may be pointed out 

 that Proptera and Paraptera, both active shells, show by my evidence 

 that they may, or may not, have this portion of the shell developed, 

 in either environment. March may be quoted in this connection on 

 the growth ol Naiades, "anterior growth is slower than the posterior, 

 as is natural in the case of an animal which has to plough its way 

 through the mud. An increase in current in which they live would 

 produce decrease in preumbonal development and so tend to throw 

 the umbo forward in those forms living in strong currents. Forms 

 with umbo thrown forward are found in canals having strong gradi- 

 ents." With regard to L. ovata, to which there are some exceptions, 

 nothing I have found may be related to the morphological facts, except 

 that shells from Lake Erie may be unmodified descendants of more 

 strenuous ancestors. Some hypothesis of evolution might support 

 this. Remembering Wilson & Clark's observation, it may be associated 

 with food conditions. Still a tendency toward greater anterior develop- 

 ment may be seen in Table III — where Eurynia, L. ovata, and Paraptera 

 are equally divided in the number of cases showing superior develop- 

 ment of the anterior hinge-line. 



{d) On the Length of the Posterior and Anterior Hinge-lines. 



As the posterior hinge-lines join the posterior portions of the shell, 



we should xpect to find them best developed in the localities where 



these are best developed. This seems to be a reasonable correlation 



ANN. CAR. Mus., xHi, 13, Dec. 3, 1920. 



