Ortmann; Families and Genera 01 Najades. 261 



and water-tubes well developed, septa rather distanl in the male, 

 In the female, only the outer t;ill is marsupial, wiili crowded septa, 



while in the inner t;i!l the septa are like those of the male. 



When gravid, the water-tubes (ovisacs) of the miter ^ill swell mod- 

 erate^ , ami a>sume a lanceolate shape, and consequently the placenta? 

 have this shape. The edge o\ this gill does not distend. The eggs and 

 placenta? are pink or red, and the placentae preserve their shape, and 

 are discharged whole. This discharge has been actually observed 

 in two cast's (July 25, 19 10). 



Glochidia of the usual shape, rather small, semioval, without hooks. 13 

 Color of soft parts very characteristic. All specimens seen had a 

 peculiar, pale orange ground-color. The foot, margins of the mantle, 

 and adductors were darker, often deep orange. The abdominal sac 

 i> whitish, the palpi and gills pale grayish brown, the gills with more 

 or less ol the orange hue. The color may be more or less intense, 

 but some shade of orange always prevails. When gravid, the red 

 color of the placenta? contained in the gills blends with the orange of 

 the latter, producing a very peculiar lilac tint. 



Plethobasus cooperianus (Lea). 



Only two specimens with soft parts have been observed, the one 

 from the Ohio River in Beaver Co., Pennsylvania, the other from the 

 Ohio at Parkersburg, Wood Co., West Virginia. Both proved to be 

 females. 



The anatomy of this species is practically identical with that of 

 P. ccsopus agreeing in all particulars, chiefly also in the peculiar color 

 of the soft parts. This color is so characteristic that it alone suggested 

 to me the relationship of cooperianus and ccsopus, which was confirmed 

 by the subsequent anatomical investigation. No gravid females have 

 been observed, and nothing is known about the color of the placentae 

 and the shape of the glochidia. 



Genus Pleurobkma Ratinesque. 1820. 

 Simpson, 1900/', p. 745 (amended). 

 Shell rounded, subquadrate, oval, or somewhat elongate, but then 

 oblique, with the beaks placed anteriorly. Beaks generally somewhat 



13 The glochidia were observed only in one case, in one of the discharging females, 

 and apparently were discharged prematurely (a phenomenon observed by others). 

 They were young and incompletely formed (soft), but their shape could be made out. 

 The -ize was about 0.18 mm., but it was impossible to make exact measurements. 



