ortmann: south American naiades. 



493 



posterior in the left valve, which may become very small, and sometimes an ac- 

 cessory third posterior tooth may develop in the right valve. Thus the pseudo- 

 cardinals are quite variable in number, shape, size, compression, and development 

 of rugosities. 



Cavities of shell and beaks shallow. Nacre blueish white or white, often 

 discolored and with lurid tints (grayish purple) toward the beak-cavities, iridescent 

 posteriorly. 



Anterior adiluctor-scar distinct and impressed, subtriangular. Anterior 

 retractor-scar separated from it, small, round, deeply impressed. Anterior pro- 

 tractor-scar connected with adductor-scar, forming a posterior process of it. Pos- 

 terior adductor-scar shallow, subtriangular, with an ui)per process formed by the 

 posterior retractor-scar. Pallial line distinct. Dorsal muscle scars a few, lying 

 in a line in the beak-cavity. 



Anatomy.— Anal opening slit-like, shorter than branchial opening, closed 

 above; closed part about twice as long as open part, no supra-anal formed. Bran- 

 chial opening separated from anal by a solid connection of the inner mantle-edges. 

 Inner edge of anal almost smooth, that of branchial with distinct papillae. In 

 front of the branchial the mantle-edges are unconnected. Palpi subtriangular, 

 a little longer than wide, their posterior margins connected at base. 



Gills long, the outer subtriangular, wider in the middle than the inner. The 

 inner not triangular, wider than the outer anteriorly, its anterior end attached 

 to the space between anterior end of outer gill and palpi, and in contact with the 

 posterior base of the latter. Inner lamina of inner gill entirely connected with 

 abdominal sac. 



Non-marsupial gills (PI. XLV, fig. lo) with few, scattered, irregularly dis- 

 posed interlaminar connections. Marsupium (PI. XLV, fig. 16) located in the 

 inner gills, occupying only a part of them, at and in front of the middle; at anterior 

 end about one-fifth of the gill remains non-marsupial, and at the posterior end 

 about two-fifths, so that the marsupium occupies two-fifths of the gill, the second 

 and third from the anterior end. This location of the marsupium is constant for 



