530 



MEMOIRS OP THE CARNEGIE MUSEUM. 



broad, shallow furrow in the anterior part of the shell, seems .to confirm this as- 

 sumption. In other respects it is impossible to decide, whether this is, or is not, the 

 (BthiopH of Von Ihering. It siwely is not the cethiops of Lea. Of the few other 

 species incidentally mentioned by Von Ihering as found in the Guahyba-drainage, 

 none can be compared with our species. Its chief characters are the subovate to 

 subtrapezoidal distinctly oblicjue shape, narrow in front, broader behind, and the 

 l)eculiar compression of the slicll in the anterior part. The beak-sculjiture and 

 color of the epidermis are also characteristic. 



Measurements. 



I cannot compare this species with any other, except D. ellipticus 8pix. The 

 general shape is yery much the same, but ellipticus seems to be more elongate 

 (height only 54 pr. ct. of length). The posterior end is slightly more pointed, and 

 the diameter is distinctly less (31 pr. ct. on the average). In our species the average 

 height is 62 pr. ct. and the diameter 39 pr. ct. In addition our shell seems to be 

 thicker and more solid, chiefl.y in old specimens, where the lower anterior margin 

 is considerablj^ and strikingly thickened, a character not mentioned in ellipticus. 



Old shells often become freakish, assuming irregular shapes. Frequently the 

 posterior part of the shell grows more strongly in length, thus rendering the shell 

 exceptionally long (as in No. 23). In such specimens the projection of the lower 

 margin is obscured, and the posterior point of the shell is very little elevated above 

 the base-line. Furthermore the shell in general is more pointed behind. Such 

 specimens also appear more swollen. However, the growth-lines clearl,y indicate 

 that, when young, these individuals had the normal shape. In other old shells 

 the whole posterior part is more developed (No. 24, PI. XXXVIII, fig. la) and is 

 deflected downward. This fact tends to preserve the general shape, but renders the 

 anterior part of the lower margin somewhat concave. 



Anatomy. — Judging from the soft parts at hand, twelve specimens are males, 

 five are barren females, and seven are gravid females. Only one of the latter con- 

 tained immature glochidia. The date of collection (January 26) apparentl.y is 

 near the beginning of the breeding season. 



