542 



MEMOIRS OF THE CARNEGIE MUSEUM. 



However, it yliould he saitl tliat of all species of tlie grou]) this one most nearly ap- 

 proaches the circular outline. It has been placed by Simpson, for this reason, in 

 the subgenus Cyclomya, but, as has been indicated already by Von Ihering, there 

 are specimens with the anterior part of the ventral margin less convex. The 

 figures of the type (Spix and Von Ihering) clearly show the oblique shape of the 

 shell, with the highest part more posterior, at, or behind, the posterior end of the 

 ligament; and there is thus no question that this species falls into the same group 

 with fontainiantis and gratus. 



^'on Ihering (1893, p. 93) emphasizes the separation of the anterior protractor- 

 scar from the adductor-scar. In my specimens, this separation is not so distinct, 

 but onl.y partial, so that we must not lay too much stress upon this character. 



Measurements. 



According to Von Ihering (1910, p. 139), this species reaches the length of 



76 mm. 



Subgenus: Cyclomya Simpson (1900). 



As has been stated, I restrict this subgenus to those shells which have a sub- 

 rotund outline, with the greatest height located at about the middle of the shell, 

 that is to say, vertically below the middle of the ligament. The posterior part 

 of the shell is thus rather short, the shell is not obliquely subtrapezoidal, as in the 

 species of the ellipticus-grou\:), but rather pentagonal in outline. The species at 

 hand is rather large and heav3% and belongs to the largest forms known in the genus. 



28. DiPLODON (Cyclomya) paranensis (Lea) (1834). 



Shells: Plate XXXIX, figs. 6, 7. 



Unio paranensis Lea, Obs. I., 1834, PI. 14, fig. 42; D'Orbigny, 1843, p. 603; 



SowERBY, XVI, 1866, PI. 51, fig. 268; Corsi, 1901, p. 451. 

 Diplodon (Cyclomya) paranensis Simpson, 1900, p. 887; 1914, p. 1284. 

 Unio nocturnus Lea, Obs. X, 1863, PI. 42, fig. 288. 

 Diplodon {Cyclomya) nocturnus Simpson, 1914, p. 1285. 

 Unio paraguayanus Von Martens, 1895, p. 34. 

 Type-locality. — Rio Parana. 



