610 



MEMOIRS OF THE CARNEGIE MUSEUM. 



lines, becoming lamellar upon the ])osterior slope and near the lower margin. There 

 are hardly an.v traces of radial sculpture. Color olive-brown, rather uniform. 

 The smallest si^eeimen has a trace of a dark radial ray upon the posterior slope, 

 otherwise there are no rays, nor indications of growth-rests. 



Hinge-line upon the whole gently curved, but the jmrt behind the beaks is 

 practically straight, chiefly in the younger specimens. The part in front is decurved, 

 but in one specimen it distinctly curves up again at the anterior end. Ligamental 

 sinus triangular, hardly as deep as wide, its anterior margin in the largest specimen 

 being vertical, in the others slightly descending backward. 



Cavity of shell and beaks rather deep, corresjionding to the obesity of the 

 valves. Nacre blueish white, with purple and green iridescence, and with very 

 faint radial striae in the younger specimens. Prismatic zone quite narrow, sub- 

 concentric with the margin. Anterior adductor-scar 'not very deeply impressed, 

 subovate, united with the scar of the anterior retractor; scar of anterior protractor 

 separated from it more or less completely. Posterior adductor-scar faint, sub- 

 ovate, with an upper triangular process formed by the ]iosterior retractor. Pallial 

 line subconcentric with the margin. 



Remarks. — I cannot find among the described species any one which agrees 

 with the present form. Its chief feature is the broadly rounded posterior end. 

 In this character and in the obliciuity it resembles the species of the patagonica- 

 group, but it differs from them in the prismatic border, which is narrow. I have 

 placed this species in the in^ona-group, although it has not the subpointed pos- 

 terior end, because I could not conveniently ])lace it anywhere else. Of other 

 species, only A. obtusa resembles it to a degree, but the latter is somewhat shorter 

 and higher (Height 64 to 69 pr. ct. of L.), and not distinctly oblicjue, with the 

 upper and lower margins more nearly parallel. Besides, A. obtum has a peculiar 

 color-pattern. 



Anatomy. — Soft i^arts of two males and one gravid female at hand. 



The structure is of the normal Anodontites-type. Pai)ilhie of the branchial 

 opening very small. Gills with the septa well developed. In the gravid female, 

 the inner gills have the usual marsujiial structure, with a swelling at the insertion 

 of the septa on the outei- (i)rimary) limb of the gill. The water-tubes contain 



