ortmann: south American naiades. 535 



males or females, the former being more subrotuiid, the latter more subov^ate- 

 subti-apczoidal. Dorsal margin gently convex, forming a more or less distinct, 

 obtuse angle with the posterior margin, the latter obliciuely descending, straight, 

 or gently curved, curving broadly around into the lower margin, forming tlie in- 

 distinctly defined, rounded, posterior end of the shell. Lower margin ascending in 

 its anterior part, gently curv(>d in the higher shells, or almost straight in those 

 more elongated, its posterior part curving up towards the posterior margin. The 

 lowest point of the lower margin (and greatest height of shell) is located well back- 

 ward, vertically below the posterior end of the ligament, or even behind this. Thus 

 the shell presents a rather oblique shape, being narrowed anteriorly, and higher 

 posteriorly, but, according to the varying height, this obliquity is more or less 

 pronounced, most distinctly in the more elongated shells, less so in those more 

 elevated, where the outline approaches the subrotund shape. 



Valves moderately but variably convex, convexity greatest upon the broad 

 posterior ridge, smallest upon the sides of the disk, where elongated specimens 

 are almost flat. Posterior slope slightly compressed, sometimes with a trace of a 

 radial furrow and a ridge, which, however, may be entirely obliterated. Diameter 

 36 to 50 pr. ct. of length. The more compressed specimens are the smaller, but 

 no distinct relation to sex can be discovered in this. Beaks moderately swollen, 

 not very prominent, located at 25 to 34 pr. ct. of the length (more anteriorly in 

 larger specimens). Beak-sculpture weakly developed, seen only in smaller speci- 

 mens, consisting of twelve to thirteen radial bars, of which the seventh and eighth 

 unite v-shapedly. The bars do not differ much in length, although the posterior 

 ones are slightly longer, the maximum being hardly over 8 mm. long. They are 

 not very sharp, but rather blunt, only the most anterior and posterior (upon pos- 

 terior slope) are somewhat finer and sharper. Often there are upon the posterior 

 slope irregular oblique wrinkles, which may be rather numerous, or may be al- 

 together absent. Lunula absent in young, but sometimes seen in older specimens, 

 short and very narrow. 



Epidermis smooth in the middle of the disk, but not very shining, covered 

 with numerous somewhat irregular, fine, concentric lines, and more widely separated 

 irregular concentric wrinkles. The concentric lines are sublamellar upon the an- 

 terior end, the posterior slope, and towards the margin, and sometimes they are 

 sublamellar all over the disk, but the latter is the case only in young individuals. 

 In old specimens they undoubtedly have been worn off. Radial sculpture generally 

 present. It may be very obscure, chiefly in younger shells, but is visible in older 

 shells, consisting of faint, blunt radial ridges, which are rather irregular, often in- 



