ortmann: south American naiades. 



525 



but not quite so far upon the rest of the slicU, and coverinf>; altogether about one- 

 third or one-fourth of the shelL There are about fifteen or sixteen radial bars, of 

 which the eighth and ninth unite, and between the latter there is another shorter 

 pair, also united in v-shape. These bars are rather sharp, those just in front of the 

 umbonal ridge are hardly broader. The most anterior bars are sometimes slightly 

 granular, occasioned by the growth-lines cutting across them. There are generally 

 close to the beaks a few additional radial bars upon the posterior slope, and below 

 then some oblique, irregular wrinkles, sometimes crossing the former, so as to 

 form V-shaped angles (forming the posterior system of re-entering angles of Lea). 

 Lunula present, short, narrower or wider. 



Epidermis with numerous, fine, concentric growth-lines, sublamellar on pos- 

 terior slope, and in the larger specimens with traces of radial lines, chiefly in the 

 front part of the shell. C'olor brown to blackish brown, without color-markings. 



Hinge-line gently curved. Ligamental sinus over the posterior third of the 

 laterals. Lateral teeth gently curved, thin, one in right, two in left valve. Pseiido- 

 cardinals obliquely descending, rather long, compressed and thin, two in right 

 valve, the posterior more elevated, with serrated edge. In the left valve also 

 two pseudocardinais, the anterior larger, serrated, the posterior much smaller, 

 sometimes rudimentary. 



Cavity of shell and beaks moderate. Nacre whitish. Anterior adductor- 

 scar moderately impressed. Anterior retractor-scar separated, and anterior pro- 

 tractor-scar united with adductor-scar. Posterior scars indistinct, united. Dorsal 



scars in beak-cavity. 



Measurements. 



Remarks. My specimens are rather uniform in shape and proportions. There 

 is some variation in the posterior part of the ventral margin, which may ascend a 

 little more decidedly, giving to this margin a more distinct lower projection, and 

 placing the posterior point of the shell a little higher above the base-line. In 

 my largest specimen, all angles (uiJi)er posterior, posterior, and that of lower 

 margin) are more rounded. Tlie beak-sculpture may be more or less distinct and 

 sharp, and the bars vary somewhat in length. 



