Ortmann; Families and Genera of Najades. 259 



also mentions this character, I think we may take ii .1- settled that 

 this form does not posM^ss .1 supra anal. A- regards the marsupium, 

 I can onlj >a\ thai in .ill lour females a1 hand onl} the outer gills show 

 the marsupial structure. 



In conclusion there is nothing remarkable in the structure i»t the 

 soft parts. The branchial opening is unusually large, and ha- papillae 

 on the inner edge, while the large anal has a practically smooth inner 

 edge. The palpi are normal, and their posterior margins are united 

 for one-third to one-half of their length. The gills, conforming to the 

 shape of the shell, are rather shorl and wide, and their anterior ends 

 .ire normal, the diaphragm is complete, and the inner lamina of the 

 inner gills is free from the abdominal sac. except at the anterior end. 

 The gills have the usual structure, and the female marsupial structure 

 is found only in the outer ijills, where the septa are much more crowded, 

 and the water-tubes much narrower, than in the non-marsupial gills. 



A gravid female has never been found, and consequently the glo- 

 chidia remain unknown. 



The color of the soft parts is grayish or brownish white, with ex- 

 ception of the margin of the mantle, which is brownish, and becomes 

 deep black in the region of the branchial and anal openings. 



Genus Plethobasus Simpson. (1900.) 

 Simpson, K)oob, p. 764 (as section). 



Shell rounded, ova), or slightly elongate, with moderately elevated 

 beaks, and moderately deep beak-cavities, and well developed hinge- 

 teeth. Outer surface sculptured with nodules or tubercle-, which 

 often are transversely elongated. Epidermis yellowish to brown, 

 without distinct rays. Beak-sculpture rudimentary, consisting (as 

 far a- known) of a few concentric ridges, which do not extend upon the 

 disk. Nacre whitish or pinkish. 



Sofl parts primitive in structure. Anal separated from the supra- 

 anal by a short mantle-connection. Inner lamina of inner gills free. 

 Only the outer gills are marsupial in the female, in other respects the 

 gills have the usual structure. When gravid, the outer gills swell 

 moderately, and their edges do not distend. Placenta- lanceolate 

 (leaf-like i and compressed; they are rather persistent, and are dis- 

 charged whole. Glochidia small, semioval, without hooks. 



Type P. asflpiis (Green). 



