538 MEMOIRS OF THE CARNEGIE MUSEUM. 



these differences are so slight, that they do not furnish an exact criterion for the 

 distinction of the sexes. 



Anatomy-'^. — The soft parts of eleven males and six females have been in- 

 vestigated. 



Color of distal part of foot blackish, this color sharply set off from the pale 

 basal part. Rest of soft parts whitish. Outer edge of anal opening (including 

 the closed part) and of branchial, black; this color running forward beyond the 

 branchial opening for a short distance. 



Anal opening closed above; closed part much longer than the open, the latter 

 slit-like, a little shorter than the branchial opening, and separated from it by a 

 solid mantle-connection. Branchial opening with small, but distinct jjapillse. 

 Palpi rather large for the size of the species, subtriangular, with curved lower 

 margin ; posterior margins connected only at base. 



Gills moderately long and rather wide. Outer gill with strongly convex lower 

 edge, its widest part a little back of the middle, and about as wide as the inner 

 gill in its posterior part. Anteriorly it becomes narrower, and its anterior end is 

 near the highest point of the mantle-attachment-line. Inner gill with the inner 

 lamina entirely connected with abdominal sac, its edge nearly straight in the an- 

 terior part, where it is much wider than the outer gill. Anteriorly it is slightly 

 narrowed, and its anterior end is immediately behind the palpi. Structure of 

 non-marsupial gills (PI. XLVIII, fig. 3rt) somewhat unusual, with rather crowded, 

 irregular interlaminar connections, forming weak, interrupted septa; these con- 

 nections are more frequent toward the margins of the gills. Inner gill of the fe- 

 male marsupial. Its interlaminar connections (PI. XLVIII, fig. 36) are more 

 crowded, and heavier, arranged in interrupted rows running parallel to the gill- 

 filaments, forming incomplete septa. The marsupium occupies only a part of the 

 gill, leaving a small section at the anterior end free, a somewhat larger one (but 

 hardly one-fourth) at the posterior end free also, so that the marsupium is located 

 in the middle of the gill, and slightly more anteriorly than posteriorly. 



In this species, the structure of the marsupial and non-marsupial gills is more 

 alike than usual as regards the arrangement and frequency of the interlaminar 

 connections, but in the marsupium the connections are decidedly heavier and 

 stronger, and form more distinct, although interrupted, septa (See PL XLVIII, 

 figs. 3rt and 35). 



-"The anatomy of D. foutaininnus has been discussed by \'on Iliering (1893, p. 90), but the finer 

 structure of the marsupium has not been described. The glochidia are said to be subtriangular, without 

 hooks. Tlieir length is 0.4 to 0.5 mm. Thus tliey are unusually large for the genus, and belong to the 

 largest known among all Naiades. 



