Grier: Morphological Features of Mussel-shells. 147 



Hazey (i6) deals with Unio pictoriini. He states that in rapidly 

 flowing brooks the shells become long, straight, narrow, with narrow 

 growth-lines and rounded extremities, while in the quiet waters of 

 the main stream (Danube) is found a high, flat form, with broad 

 lines of growth, rounded posteriorly, and with short anterior border. 

 He further remarks that transitional forms representing this species 

 occur in places intermediate between the swift brooks and the quieter 

 river, and points out that typical Unio pictorum is developed in 

 stagnant water, but in swifter currents becomes elongate, producing 

 the variety longirostris. He shows that the "beak" obtains its 

 greatest growth in swiftly flowing streams. An excellent paper by 

 H. VVallengren (56) deals at some length with the matters which the 

 two previously mentioned writers discuss. 



With the appearance of the paper of H. Sell (45) there began a 

 revival of interest in this particular field of investigation, which has 

 continued to the present time. As the result of extensive comparisons 

 Sell concludes that in mussels found in certain lakes and rivers the 

 anterior portion of the shell is thicker in the latter and weaker in the 

 lakes. He associates a long rather than a curved lower border with 

 strong currents, regarding it as giving greater protection. Lake 

 forms are comparatively inflated and have protruding growth-rings. 

 He accounts for the presence or absence of the growth-rings by the 

 degree of the disturbance of the water. Forms from still water are 

 somewhat more symmetrical in outline, and he states as a general 

 principle that the size of Unios seems to be proportionate to the size 

 of the body of water in which they are found. 



O. Buchner (5) observes that Anodonta sp. reaches its greatest 

 development on muddy bottoms, becoming compressed in brooks. 

 This alteration in the shape of the shells, resulting in the production 

 of varieties, he is inclined to attribute to nutritive conditions. He also 

 recognizes transitional forms originating in the diff"erent environments. 

 W. V. Israel (22, 23, 24) besides corroborating the statements made 

 by the writers already quoted, dwells at length upon Unio crassiis, 

 noting that it is shorter in brooks than in quiet waters (cf. V. Hueber) 

 in which it becomes larger, with a curved inferior border. In strong 

 streams and cataracts, especially where gravel is found, he observes 

 that the mussels become stronger in structure, shortened, and rounded. 

 He distinguishes three varieties of Unio pictoriini formed in the way 

 indicated: (a) the common typical form of the larger rivers; (b) a 



