ortmann: south American naiades. 



483 



eessory ridges of the left valve. Similar, but rather varial^le, conditions are seen 

 in the younger individuals and the specimen from S. Antonio. The rule is that 

 there are three teeth in the right, but only two in the left valve. 



Cavity of the shell moderate, that of the beaks shallow. Nacre shining, snow- 

 white in my specimens, in two (No. 1 and the one from S. Antonio) with a faint 

 salmon blush in th<> cavity of the shell. Anterior adductor-scar deeply impressed 

 even in young specimens, irregularly rounded or broadly subelliptical. Anterior 

 retractor-scar located above it, separated from it, deep, small. Anterior protractor- 

 scar connected with it. Posterior adductor-scar distinct, but much less impressed, 

 sul')0vate or subtriangular, with a rounded or triangular appendix above, formed 

 by the posterior retractor-scar. Pallial impression distinct. Dorsal scars five or 

 six, in cavity of beaks, placed irregularly, or in an oblique line. 



There are no sexual differences in the shell. 



In Lea's text there is apparently an error with respect to the height, since the 

 measurement would yield the absurd figure of 89 pr. ct. of the length. According 

 to the figure Lea had a specimen with the beaks a httle more anterior than any of 

 mine, but otherwise rather closely agreeing with them. 



Remarks. — This species is related to those of the hyloeus-group, chiefly to 

 D. guaranianus, but it is easily distinguished by its long-ovate shape, with pointed 

 posterior end, and by the beak-sculpture, which, although of the same type, does 

 not extend so far ujjon the disk. A good character is furnished also by the color 

 of the epidermis and its concentric bands, though we have seen that this pattern 

 is at least indicated in young D. hasemani. Lea compares D. trifidus with D. 

 bttrroughiamis and parallelopipcdon, but there hardly is any close relationship to 

 the former, and but a superficial resemblance to the latter. I think it belongs to 

 the hyloeus-group, and may be characterized as an extremely' elongated member 

 of that group, in which the beak-sculpture is not quite so fully developed as in 

 the other species. 



Anatomy (Text-fig. 1, 11. 455). — The three smallest of my specimens could not 

 be satisfactorily examined to ascertain the sex, but the gills did not show the mar- 



