ORTMANN: FAMILIES AND GENERA'OF NAJADES. 259 
also mentions this character, I think we may take it as settled that 
this form does not possess a supra-anal. As regards the marsupium, 
I can only say that in all four females at hand only the outer gills show 
the marsupial structure. 
In conclusion there is nothing remarkable in the structure of the 
soft parts. The branchial opening is unusually large, and has papille 
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 short and wide, and their anterior ends 
are 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 gills, 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, 1900), p. 764 (as section). 
Shell rounded, oval], or slightly elongate, with moderately elevated 
beaks, and moderately deep beak-cavities, and well developed hinge- 
teeth. Outer surface sculptured with nodules or tubercles, which 
often are transversely elongated. Epidermis yellowish to brown, 
without distinct rays. Beak-sculpture rudimentary, consisting (as 
far as known) of a few concentric ridges, which do not extend upon the 
disk. Nacre whitish or pinkish. 
Soft 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. Placente lanceolate 
(leaf-like) and compressed; they are rather persistent, and are dis- 
charged whole. Glochidia small, semioval, without hooks. 
Type P. esopus (Green). 
