600 



MEMOIRS OF THK CARNEGIE MUSEUM. 



Hinge-line practically straight in young shells; in older shells it curves gently 

 down under the beaks, and up again at the anterior end, and the posterior end 

 curves gently down, thus forming a slight S-curve. Ligamental sinus triangular, 

 not deeper than wide, varying with age (shallower in yoiuig shells), its anterior 

 margin running obliquely backward in the young, and vertically in older individuals; 

 its lower point maj^ be sometimes directed forwards. 



Cavity of shell and of tlie beaks rather deep, corresponding to the obesity 

 of the shell. Nacre white; in young shells blueish white, in older shells somewhat 

 inclining to cream-color, always extremely glossy, silvery, and iridescent toward the 

 margins, with fine, straight, and irregular radiating lines. Along the margin there 

 is a rather narrow nacreless (prismatic) zone, relatively wider in young specimens. 

 This zone is subconcentric with the shell-margin, widest in the middle, gradually 

 narrowing towards the ends, but nowhere suddenly or markedly changing its width. 



ABterior adductor-scar and anterior retractor-scar united, not deep, irregularly 

 ovate or elliptical; anterior protractor-scar connected with adductor-scar, or, 

 in old specimens, more or less (sometimes distinctly) separated. Posterior ad- 

 ductor-scar faint, subovate, the posterior retractor-scar forming an upper })ro- 

 jection thereof. No dorsal scars. Pallial line subconcentric to margin. 



Measurements. 



The variety (or species )spixi D'Orbigny from the lower Parana at Corrientes 

 grows larger, reaching the length of 81 mm. according to Von Ihering, and of 90 

 mm. according to D'Orbignj'. 



Remarks. — This shell is easilj^ recognized by the general shape and proportions, 

 by the rather narrow prismatic border, and by the dull, greenish color of the epi- 

 dermis. That trapezea and rotunda are only the old and the young stages of the 

 same species, is conclusively shown by our material, chiefly by the sets from Cidade 

 da Barra. The old specimens have the beaks a little elevated' above the hinge- 

 line, and the hinge-line is gently curved ; while the young specimens have the point 



