618 MEMOIRS OF THE CARNEGIE MUSEUM. 



over, my largest specimens of ruhicunda differ in this from the smallest specimens 

 of patagonica (of about the same size), but not very strikingly. 



# 



Much stress has been laid in the original descriptions of A. ruhicunda and 

 pazi upon the red color of the nacre. According to my material this is extremely 

 variable. In some specimens the color does not differ from that of A. pataf/onica, 

 being whitish. In others it is more or less tinted with red or purple, and then also, 

 generally, the prismatic border is dull purplish gray. But this also is not a constant 

 character. 



Pilsbry's/c^/x is said to be characterized by its light, yellow-green epidermis, 

 which has radiating or irregularly zig-zag lines of green. I see traces of this in 

 some of my specimens from Uruguayana and Santa Isabel, and there are also here 

 and there black markings on the inside along the pallial line and near the muscle- 

 scars (and elsewhere), mentioned by Pilsbry as occurring in felix. These are in- 

 dividual characters. 



The epidermis of rubicunda is much as in patagonica, i.e., somewhat lamellar 

 near the margins, smoother in the middle. Hcaladform stripes are sometimes 

 indicated, but are mostl.y absent. The color in younger specimens is generally dark 

 green all over the shell, but a few are brownish. Those with intense red tints 

 on the inside are sometimes a little reddish brown on the outside. The largest 

 specirnen in the color of the epidermis is much like patagonica, brownish, inclining 

 to olive toward the beaks. 



D'Orbigny says of patagonica {rnemhranacea) that it is very variable; that it 

 is larger and thicker in the larger rivers; more elongated in lakes; and that it be- 

 comes reddish on the inside in small rivers. My specimens support the latter 

 observation, for they all are, §o far as thej^ may be called rubicunda and have red 

 nacre, from comparatively small rivers (Rio Negro, upper Uruguay, Ibicuhy, and 

 Cacequy). But, of course, this requires further study and more material. 



Anatomy: The soft parts of six males and seven females are at hand, one of the 

 latter gravid (February 5). 



The anatomy of two males of pazi has been described by Lea, but only the 

 major features have been mentioned. According to my material the soft parts 

 agree entirely with those of the typical patagonica. In the gravid female the 

 swelling of the charged marsupium is not considerable. The small eggs hang 

 loosely together, and easily fall apart. No fully developed larvae could be found. 

 A diagrammatic figure of the soft parts is given in text-figure 3 (p. 458), and the 

 structure of the gills of a barren female in cross-section is shown on PL XLVIII, 

 fig. 8. 



