576 MEMOIRS OF THE CARNEGIE MUSEUM. 



mens from Cachoeira (Giiahyba-drainagc) entirely correspond with the others 

 in these respects, and I cannot distinguish them by any other characters. 



The chief features of this species are the subcircular, or rather rounded-pen- 

 tagonal outline, the great height of the shell, the location of the greatest height 

 close behind the middle of the shell, and the flatness of the valves. If properly 

 placed, with the ligament horizontal, it is seen that the shell is distinctly oblique, 

 and that the anterior end is narrower. 



Anatomy. — Three males and three barren females are at hand. 



Von Ihering (1893, p. 69) has mentioned a number of anatomical characters, 

 but his account is not complete. 



Anal opening entirely open, large, its inner edge with distinct crenulations 

 in the lower part, otherwise smooth, separated from the branchial opening by 

 a mantle-connection. Inner edge of branchial opening with distinct, but small, 

 pai^illa?. Palpi moderately large, semicircular, briefly truncated behind, the pos- 

 terior margins not connected. 



Gills (PL XLVII, figs. 3a, h) of medium width, the inner the wider, chieflj' in 

 front; the outer narrowing anteriorly, its anterior end near the highest point of 

 the mantle-attachment-line. The inner gill very little narrowed in front, beginning 

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

 abdominal sac. 



Gills with well developed septa. Those of the male (PL XLVII, fig. 3a) and 

 of the outer gill of the female alternately stronger and weaker. This alternation, 

 however, is not very distinct, and is chiefly seen in the middle of the gill. In 

 the female (PL XLVII, fig. 36) the inner gill is marsupial, with the septa more 

 uniform and stronger, l)ut hardlj^ more crowded. At the point of union with the 

 outer lamina, the usual swelling is present, indicating the ridges projecting into 

 the water tubes. The most anterior and most posterior extremity of this gill 

 has not the marsupial structure, but the marsupium occupies nearly the whole 

 gill. 



In the specimen sectioned, the swellings of the septa are located nearly in 

 the middle. However, the part of the septa from the swelling toward the inner 

 lamina is thicker, and has more strongly developed epithelium, indicating that 

 this is the part which stretches out, when gravid. The condition seen in this 

 specimen undoubtedly is du(> in pavi to its barren character, in part to the state 

 of preservation. 



