48 



THE NAUTILUS. 



different, in the examples of the pseudo Nephronaias seen by the 

 writer. Nephronaias (s. s.) posseses an (accessory?) adductor 

 scar attached to the frontal portion of the cardinal teeth, which 

 is either absent or obsoletely marked in the second assemblage. 

 For this latter group the writer, therefore, proposes to use the 

 generic term of Actinonaias Crosse and Fischer, 1893, type U. 

 sapotalensis Lea. The female of this species has been described 

 by Dr. Ortmann (1912). Actinonaias embraces, besides the type, 

 medellinus, gundlachi, (accepting Simpson's interpretation of 

 this latter species), and others. 



Psoronaias, Crosse and Fischer (1893). This group of re- 

 markable shells, embracing crocodilarum, psoricus, semigranosus, 

 etc. , was provisionally treated by Simpson as a group of Elliptio, 

 but their remarkable sculpturing, and the deap beak cavities of 

 some of their species, led him to observe that it was possible 

 that the group should, after all, be placed in Quadrida. 



I follow von Martens, in giving generic rank as above to the 

 group. The type is Unio psoricus. To this genus we are en- 

 abled to add a species hitherto undescribed, under the name of 



Psoronaias kuxensis, n. sp., PI. VII, figs. 1, 2. 



Shell small, compressed, rough, brown, biangular. Length 

 50, height 30, diam. 17 mm. 



Shell hyperbolically rounded before, the extreme frontal 

 point below the centre. Dorsum slightly arched, descending 

 behind the ligament to the widely biangular posterior ; the 

 upper angle of which is midway the height, the lower angle 

 very little above the base, which is nearly straight. The beaks 

 are small, low, acute, approximate; and apparently, concentri- 

 cally ridged. Epidermis dark brown (olivaceous and obsoletely 

 rayed in the young), rough, the lines of growth numerous and 

 well impressed. The discs are covered with fine pustulations, 

 more pustular in front, biradially linear behind. The post 

 ridge is low, but distinctly double, making the shell biangulate 

 behind. The teeth are double in the left valve, single in the 

 right. The cardinals are deeply sulcate and stout. Laterals 

 slightly curved or nearly straight, separated by an interdentum. 

 Nacre purple, beak cavities rather deep. Dorsal scars numerous, 



