Grier: Morphological Features of Mussel-shells. 179 



the line was too curved to measure directly. In certain species the 

 work was easier than in others, but, because the results appear to be 

 uniform throughout, I am confident that no serious rnistake was made. 

 I have given at some length a detailed diiTerential description of 

 those environments, the Naiades of which we have compared. With 

 the change of environment from the river to the lake there is seen to 

 be a parallelism in the change of the morphological character of the 

 shell. As pointed out previously, shells from Lake Erie are as a rule 

 smaller, have broader growth-lines, brighter colors, and more polished 

 epidermis, and are thinner than those of the Upper Ohio. Still 

 specimens from many parts of the Upper Ohio could, even in these 

 respects, be confused with those of Lake Erie, as I found wdien de- 

 scribing new varieties from the lake (14). This necessitated careful 

 measuring for their accurate determination, leading into the investi- 

 gation I have here attempted. It will be observed that this paper 

 merely extends the similar work carried on by systematists, and 

 deduces certain laws stated in the conclusions. And of all theories of 

 evolution advanced, Lamarckian factors perhaps account for these 

 changes better than any others. Moreover the principle stated by 

 Sell (45)' — that the size of Unios seems to he proportionate to the size of 

 the body of ivater in tvhich they are found — finds strong substantiation 

 in my results as does the important principle first stated for Mollusca 

 by Haas and Schwarz (15) viz: "The same original types change,- 

 under the same biological (ecological), conditions, into identical local 

 forms: different original types change, under the same biological (eco- 

 logical), conditions into convergent, (parallel) local forms. In conse- 

 quence of sufficiently long isolation, the local forms may develop into 

 constant local varieties.'' 



XL Literature Cited with Bibliography of 

 Ecology of Naiades. 



1. Adams, C. C. The Variations and Ecological Distribution of the Snails of 



the Genus lo. Memoirs of the Nat. Acad, of Science, 1915, XII, Part 2, 

 p. 38, et seq. 



2. Allen, R. The Food and Feeding Habits of Freshwater Mussels. Biological 



Bulletin, 1914, XXVII, No. 3, pp. 138-39. 



3. Baker, F. C. On Some Variations of Cardium Edule apparently correlated 



to the Conditions of Life. Phil. Tran., 1889. 



4. Baker, F,. C. The Relation of Mollusks to Fish in Oneida Lake. Technical 

 * Public. No. 4, N. Y. State College of Forestry, Syracuse, 1916. 



