ortmann: south American naiades. 563 



beak-sculpture is generally well developed, and covers all of the shell in young 

 specimens; but in older individuals it disappears near the lower margin, and some- 

 times it is poorly developed, and present only for a short distance from the beaks 

 (form transversa Uupe), but, as stated above, the latter specimens are connected 

 with the typical form by intergrades. 



Anatomtj.— Two females are at hand, one of them gravid, and with eggs (col- 

 lected December 10).'^ 



The soft parts have been previously described and figured by myself (191 lu, 

 pp. 108, 114, PI. 6, fig. 6, PI. 7, fig. G). 



Anal opening closed above; closed i)art about twice as long as the opening, 

 the latter slit-like, separated from the branchial opening by a mantle-connection. 

 Branchial opening with small papilla?, not closed in front. Palpi rather small, 

 subtriangular, posterior margins not connected. 



Gills of the usual shape, the inner much wider anteriorly than the outer, its 

 anterior end close behind the palpi) r' Inner lamina of inner gill entirely connected 

 with abdominal sac. Non-marsupial gills with scattered interlaminar connections. 

 Marsupium of the female in the inner gill, restricted to the middle portion, al^out 

 one-fourth of the gill at anterior end, and a little less than that at the posterior 

 end being non-marsupial. Interlaminar connections of marsupium crowded and 

 numerous, forming interrupted septa, and assuming in the most central part an 

 irregularly ciuincuncial (reticulate) arrangement. 



Genus Prisodon Schumacher (1817). 



Simpson, 1900, p. 869; 1914, p! 1216. 



Closely allied to Hyria in the doubly alate shell, but differing in the absence 

 of beak-sculpture, and the strong development of a sharp posterior ridge. 



A key to the species has been given by Simpson (1914, p. 1217). 



This genus is also characteristic of Guyana and the Amazon-drainage in Brazil. 



39. Prisodon alatus (Sowerby) (1869). 

 S/ieMs.- Plate XL, figs. 1,2,3. 



Hyria alata Sowerby, XVII, 1869, PI. 5, fig. 13. 

 Prisodon alatus Simpson, 1900, p. 871; 1914, p. 1220. 

 Type-locality. — Guyana. 



-^ The sholl of this Rravid female approaches the form Irnnsmsa, with the beak-sculpture less 

 ,lev<'lope.l, but with the shajie not very elo.iKated. The other female is a normal corrugata, half-grown. 



=''The connection of this gill with the palpi described i,reviously is only accidental, and seen only 

 on one side of the larger female; on the other side, and also in the smaller female, the structure is normal. 



