564 



MEMOIRS OP THE CARNEGIE MUSEUM. 



NexD Locality. — Sand-bar of Rio Tapajos, Santarcm, Para, Brazil (J. D. Hase- 

 man coll., December 6-12, 1909). About sixty si)ocimcns, all dead shells. 



Distribution. — The discovery of this species, hitherto apparently only known 

 from Guyana, in the Amazon-drainage is interesting. 



The shell is easily recognized by the relatively short triangular shape, and the 

 excessive development of the anterior and posterior wings. The outline varies a 

 good deal, being longer or shorter, and the lower posterior end of the shell may 

 be blunt or somewhat pointed. The obesity is also variable. The wings are 

 more or less developed, and the posterior one may be longer or shorter, sometimes 

 as long as the rest of the shell, narrower or wider, pointed or blunt. It is directed 

 straight backwards (forming a straight continuation of the hinge-line), chiefly so 

 in young specimens; but in older ones it may be directed more or less upward, so 

 that the hinge-line becomes concave. Upon the posterior slope there are sometimes 

 a few (three to five) parallel folds or wrinkles, but most specimens lack these. 



The pseudocardinals (PI. XL, figs. 2c, d) of this species are very long and com- 

 pressed, the posterior of the right valve is longer than the anterior, and extends 

 far forwards, so as to be placed in its anterior part below the anterior. In the 

 left valve there are also two iiseudocardinals, the posterior shorter than the anterior, 

 and extending onlj- with its anterior end below the anterior. The upper and inner 

 face of the posterior right pseudocardinal in its posterior part, and the space be- 

 tween the two teeth in the left valve, has distinct, parallel, and subvertical ridges. 

 The lateral teeth (one in right, two in left valve) are only ol^liquely and granulately 

 corrugated, but do not possess parallel vertical ridges. Thus in the structure of 

 the pseudocardinals this species differs from other species of the genus (I have been 

 able to compare P. castclnaudi, obliquus, brownianus), where these teeth are shorter, 

 and the posterior one in each valve is split into several parts, without developing 

 vertical ridges. 



My specimens are of all sizes, but none of them reaches that of Sowerby's 

 figure, which measures, from tip to tip of the wings, 115 mm., and from tip of an- 

 terior wing to lower posterior end of shell 115 mm. 



Measurements. 



For obvious reasons (odd shape of shell) measurements could not be made here 

 in tlie usual way. It also should be noted that the wings of the largest specimens 



