THE NAUTILUS. 61 



mediate forms (ag) belong to the streams of moderate size. 

 Intergrades are frequent. 



Anatomy. ' 



All these shells have the same, and an extremely characteris- 

 tic and unique structure of the soft parts, so that there is not 

 the slightest question that they belong together. I have ex- 

 amined the soft parts of some 200 specimens in the field, and 

 over three dozens have been preserved in alcohol, and have 

 been examined at leisure in the laboratory. They include rep- 

 resentatives of the three main varieties, and of practically all of 

 the individual variations. 



Gravid females have been found on the following dates: May 

 11, '13; May 15, '18; May 16, '13; May 20, '13; May 20, '14, 

 May 22, '14; May 25, '14; July 5, '13; July 9, '18; July 10, 

 '13; July 18, '13; July 14, '13. Glochidia have been observed 

 on May 20, '14 (immature), and July 14, '13. Thus this spe- 

 cies evidently is a summer breeder (tachytictic). 



The soft parts are those of the genus Fiisconaia: the supraanal 

 is separated from the anal by a very short mantle-connection, 

 which is absent (or torn?) in rare cases. Inner lamina of inner 

 gills free from abdominal sac. All four gills are marsupial. 

 Placentae well developed and subcylindrical. 



Branchial opening with well developed papillae, anal with 

 distinct, but small papillae. Falpi subfalciform, posterior mar- 

 gins connected at base only. 



While thus the Fusconaia structure is typically developed, 

 this species is quite unique in its color. This concerns chiefly 

 the color of the gonads, eggs, and placentae. 



The soft parts are often uniformly pale, whitish, but may 

 shade to orange, and the orange is most prominent on foot, ad- 

 ductors, and mantle-margin; but the paler tints prevail, and 

 often the orange is replaced by yellowish or brown. The gills 



' In Nautilus, 28, 1914, p. 31, I have described the anatomy of " Pleuro- 

 bema fassinam.''' This is a mistake: the shells examined belong to Pleu- 

 robema all right, but are the form known as U. argenteiis Lea, which belongs 

 to the oviforme-group, and should be called : Pleurobema ovi/orme argenteiim 

 (Lea). These will be treated more fully elsewhere. 



