ortmann: south American naiades. 539 



26. DiPLODON DECEPTUS (Simpson) (1914) 



Shells: Plate XXXIX, figs. 1, 2, 3, 4, 5; Anatomy of gills: Plate XLVII, fig. 1; 



Section of gills: Plate XLVIII, fig. 4. 



Unio gratus Von Iherin(! (nmi Lea), 1893, p. 92. 



Diplodon iCydomya) Jontainianus deceptus Simpson, 1914, p. 1281. 



Type-locality. — Guahyba, Brazil. 



Other Locality. — Rio Guahyba, Porto Alegrc, Rio Grande do Sul, Brazil (Von 

 Ihering) . 



Localities Represented in Carnegie Museum. — Rio Guah>,'ba, Porto Alegre, 

 Rio Grande do Sul, Brazil (J. D. Haseman coll., January 24, 1909) two shells 

 and one right valve (Topotypes). Rio Jacuhy, Cachoeira, Rio Grande do Sul, 

 Brazil (J. D. Haseman coll., January 26, 1909) ten specimens with soft parts, 

 males and females. 



Distribution. — Drainage of the Rio Guahyba and its tributary, the Rio Jacuhy, 

 in Rio Grande do Sul. 



There is no question that my specimens belong to the form called by Von 

 Ihering gratus, and by Simpson deceptus. Von Ihering comjiares his D. gratus 

 with D. fontainianus, and says that the former much resembles the latter in shape, 

 but that the epidermis of D. gratus is brown, shows radial sculpture, and is smoother; 

 that the beaks are more anterior (24 to 31 pr. ct., average 27 pr. ct., against 29 to 

 39 pr. ct., average 34 pr. ct. in D. fontainianus); that the pseudocardinals of D. 

 gratus are shorter and more stumpy ; and tliat the posterior retractor-scar is always 

 connected with the adductor-scar. 



All these characters hold good for my specimens, and they distinguish this 

 form also from the real D. gratus of the Uruguay River, except that the location 

 of the beaks does not differ so much (in true D. gratus it is 25 to 34 pr. ct.). 



Simpson's D. deceptus has been described as being unevenly obovate, subin- 

 flated, rather solid; pseudocardinals shorter, more stumpy, and much split. This 

 also fits the present form, and I have no doubt that D. deceptus is the same shell 

 as that called D. gratus by Von Ihering. 



The chief differences between D. deceptus and D. gratus are the following: 



1. D. deceptus has a greater tendency toward the obliquely elongated shape, 

 with subtrapezoidal or subovate outline. The height varies from 65 to 78 pr. ct. 

 of the length, while in D. gratus it ranges from 72 to 85 i)r. ct. 



2. The ei)idcrmis of D. deceptus is alwa.ys brownish, with hartlly any green in 

 it, and not blackish green. It is smoother, since the fine concentric lines are not 



