ortmann: south American naiades. 601 



of the beaks immediately above the hinj2;e-liiie, and have tlie latter straight. Speci- 

 mens of intermediate size intergrade in these characters. 



Anatomy. — The soft parts at hand are not in good condition. However, the 

 structure of the gills can be made out, and according to the alternation of stronger 

 and weaker septa, all four specimens at hand are males. 



In other respects the usual structure of the genus is seen. Anal ojicning en- 

 tirely open, separated from the branchial by a mantle-connection. Branchial 

 opening with distinct, but small papilla?. Palpi small, subcircular, with a short 

 truncation at the posterior end; the posterior margins not connected. Inner lamina 

 of inner gill entirely connected with abdominal sac. 



4. Group of Anodontites triyona. 

 Simpson, 1914, p. 1441. 



Shell not very oblique, rather elongated, subelliptical, or subovate, narrowly 

 rounded, or somewhat pointed behind. Epidermis more or less shining, and not 

 uniformly and densely covered with wrinkles, although such are present here and 

 there. Prismatic border narrow, of nearly equal width. 



This group is poorly defined. Its chief character is the rather elongated shell, 

 somewhat pointed behind, or narrowly rounded, and not distinctly oblique. The 

 comparatively smooth epidermis is another noticeable feature, but still there are 

 species, which have sublamellar, concentric striae, at least in parts of the shell. 

 The color of the epidermis is not so sombre and dull as in the preceding groups. 

 The narrow prismatic border seems to be constant. 



53. Anodontites TRiGONA (Spix) (1827). 

 Anodon trigonum Spix & Wagner, 1827, p. 29. PI. 22, fig. 2. 

 Glabaris triyonus Simpson, 1900, p. 92S. 

 Anodontites triyonus Simpson, 1914, p. 1441. 

 Anodon moretonianus Sowerby, XVII, 1867, PL 9, fig. 20. 



Doubtfully synonymous : 

 Anodon georgince Griffith, 1834, p. 595 (index), PL 19, fig. 3. 



A. moretonianus Sowerby undoubtedly is this species. Simpson (1914, p. 1431) 

 is mistaken in placing it with ^1. trapcsialis. Sowerby 's moreloniajius is not A. 

 mortoniana of Lea. 



A. r/eo/-f/(na; Griffith, from "rivers of Paraguay," also seems to be this species. 

 The figure given I.e. is shorter and higher, but the characteristic shape and the 

 radial ribs (although too much emphasized) well agree with it. It is also from a 

 region, where trigona is known to occur. For this species, Simpson (1900, p. 927; 

 1914, p. 1440) creates a separate group. 



