ortmann: south American naiades. 523 



Remarks.— This species somewhat resenibles D. luckolus, but is considerably 

 smaller, has finer and shorter beak-sculpture, and, when old, has more simple, 

 less dissected pseudocardinals, of which those of the right valve are more nearly 

 equal. The young shell (PI. XXXVII, fig. 6a) has rather a siibrhomboidal shape, 

 exactly as D. lacteolus, which is due to the better development of the angle between 

 the upper and posterior margins, which appears as slightly elevated (alate). Thus 

 the young shell is slightly higher in its posterior section, with the posterior end of 

 the shell less elevated above the base-line, giving to the whole shell a slightly oblique 

 apiiearance. But this juvenile shape is sooner or later obliterated, the posterior 

 wing disappearing, and the posterior end becoming more tapering, giving to the 

 shell a rather subovate outline, with the posterior end subpointed; but this point 

 is never very distinct. The outline of Von Ihering's grceffcanus (PI. 4, fig. 8) comes 

 very near to the normal shape of the old shell of D. mogxjmirim. The brownish 

 color of the epidermis of our species is also characteristic, and the complete absence 

 of distinctly greenish tints is to be noted. 



Anatomy. — I have a great number of the soft parts of males and of barren and 

 gravid females, the latter partly with eggs, partly with glochidia in various stages 

 of development. Of fifty specimens the anatomy has been investigated more 

 closely. For the breeding season the date of collection (August 28) should be 

 noted. 



Color of soft parts brownish white. 



Anal opening slit-like, closed above; closed part not quite twice as long as 

 the open part. Anal about as long as the branchial, separated from it by a solid 

 mantle-connection. Branchial opening with distinct papillae. Palpi moderate, 

 subtriangular, posterior margins connected for about one-third of their length. 



Gills (PI. XLVI, fig. 5a, h, c) rather wide, the outer one subtriangular, pos- 

 teriorly slightly wider than the inner; the inner one subtrapezoidal, anteriorly 

 wider than the outer, its anterior end immediately behind the palpi. Inner lamina 

 of inner gill entirely connected with abdominal sac. Non-marsupial gills (PI. 

 XLVI, fig. 5a) with few, scattered interlaminar connections. The marsupium 

 (PI. XLVI, fig. Sb) of the females located in the inner gills, but occupying only 

 a part, anteriorly leaving free not quite one-third of the gill, and posteriorly hardly 

 one-fourth, so that the marsupium gravitates slightly toward the posterior section 

 of the gill. In young females, the marsupial part (PI. XLVI, fig. 56) is much 

 smaller, and lies distinctly behind the middle of the gill. Structure of the mar- 

 supium (PI. XLVI, fig. 5c and PI. XLVHI, fig. 2rt, b) quite peculiar, consisting of 

 uninterrupted septa, forming well isolated water-tubes. An interrupted or reticu- 



