ortmann: south American naiades. 509 



Remarks. — Among our specimens the single valve comes nearest to the measure- 

 ments given for U. rhuacokus. D'Orbigny himself admits that this may be only 

 a more elongated form of charruanus, and hisj'aba, united by him with charruanus, 

 is even more elongated. There is no reason for keeping rhuacokus separate, since 

 the proportional length is said to be the only difference. 



The variations in the shape of the shell are, indeed, very great. Younger 

 specimens are, however, more uniform; they are more elongated, and their normal 

 shape is well rendered in D'Orbigny 's figures oi faba (PL 71, figs. 12-14), and in 

 the figures of aHhiops and parcus. The latter undoubtedly is a young shell, but 

 cethiops also belongs here. Specimens like those measured under Nos. 12 and 9 

 resemble this very much, except that they are a little more swollen, and our smallest 

 (No. 14) looks very much like parcus, but it is less elongated. 



In the description and the figures of D'Orbigny only the color of the epidermis 

 is not exactly as in our specimens. D'Orbigny describes it in charruanus as well as 

 rhuacokus as brownish green, and figures it as lighter or darker brown, while the 

 color oi faba (PI. 71, figs. 13, 14) is blackish, agreeing better with our specimens. 

 However, our young specimens have a color, which may be called brownish green, 

 or rather "olive-brown." Lea's cethiops is said to be black. 



The chief characters of this species thus seem to be the subtrapezoidal, rather 

 elongate, straight shell, with a moderately sharp posterior point, located only 

 little above the Imse-line; the somewhat swollen shell, with a moderate and blunt 

 posterior ridge, and the brown to blackish color of the epidermis. The chief varia- 

 tions are found in the length of the shell, and the somewhat higher or lower position 

 of the posterior end. 



There is no question that hidalgoi Haas and fortis Marshall are this species. 

 The former is founded upon two specimens, the latter upon a single one, which 

 certainly represent individual phases. Specimens greatly resemliling these are 

 among my material. 



Anatomij. — I have the soft parts of eleven males and six gravid females, three 

 with eggs, three with glochidia, one of these discharging. 



Anal opening slit-like, closed above, the closed part about four times as long 

 as the opening; the latter shorter than the branchial opening. The two openings 

 are separated by a solid mantle-connection. Branchial with small papillae. Palpi 

 subtriangular, their posterior margins connected near the base. Gills posteriorly 

 of about the same width, but anteriorly the inner is wider, and its anterior end is 

 immediately behind the palpi. Inner lamina of inner gill entirely connected with 

 abdominal sac. Non-marsupial gills with scattered, short, interlaminar connec- 



