l6o THE AMERICAN NATURALIST. [Vol. XXXVIII. 



The valves of predacious fishes seem to be the heavier and 

 more strengthened by muscular thickenings. This may be 

 because the feeding habits of such fish would be liable to injure 

 the more delicate membranes. 



The valves vary widely in different genera and in different 

 species of the same genus. The generic variation depends on 

 general shape, size and appearance; the specific on papillae, 

 muscular thickenings and pigment. The general type of a fam- 



ily, notwithstanding, is very characteristic, except, perhaps in the 

 minnows, in some of which the papillae form valve occurs. By 

 general type, the general appearance is meant ; not the crescen- 

 tic or U-types only. Individual variation seems comparatively 

 slight and generally resulting from injury. If one individual 

 shows a certain arrangement of pigment, papillae, and central 

 thickening, it will, so far as it has been possible to ascertain, be 

 found in the rest of the species and nowhere else. For con- 

 venience of comparison and explanation the writer has, where 

 enough species and specimens were available, arranged results 

 in synoptical form. 



The writer desires to thank Dr. Burt G. Wilder and Dr. H. 

 D. Reed for valuable hints and use of specimens and, more 

 especially, for their kind interest and encouragement. 



