i6o Studies in Geology, No. 3 



"Shell thin, compressed, inequilateral; beaks low, hardly 

 interrupting- the dorsal profile, but sharp and almost 

 pustular, ligament rather long, hinge delicate but normal; 

 surface polished, with rather distant, fine, concentric im- 

 pressed lines ; the posterior dorsal slope with sparse, sharp, 

 little elevated concentric lamellae : near the beaks the shell 

 is smooth ; interior with a faint anterior elevated ray which 

 separates the adductor scar from the anterior part of the 

 long, high pallial sinus, which is wholly confluent below ; 

 the interior more or less obscurely radially striate. Lon. 

 12.5 ; alt. 7.5 ; diam. 2 mm. A thin and delicate species with 

 no observable flexure or ridge on the posterior end, and 

 whose especial characteristic is the high dorsal profile, 

 behind the beaks." — Dall, 1900. 



The Peruvian forms, though delicate and thin, are some- 

 what heavier than the usual specimen from Bowden, the 

 chief evidence of greater weight being found in the pro- 

 portionally stronger hinge armature, in which the elements 

 are exactly similar, but the development of the individual 

 teeth is somewhat greater, and the armature of the inner 

 dorsal valve-margins consists of a heavier plate than in the 

 Bowden specimens. Otherwise the forms are identical, and 

 Ball's description covers every point worth noting. The 

 anterior margin is produced, rounded ; the ventral margin, 

 flat to very slightly emarginate, ascends somewhat; the 

 characteristic posterior marginal angle is prominent ; the 

 posterior dorsal margin is high in profile, and curved, as 

 noted by Dall. The concentric sculpture follows the mar- 

 ginal outline, and is hence flat to slightly flexed upward 

 mesially. A complete shell from Zorritos has the following 

 dimensions: length, 15.5; height, 8.5; diam., 3 mm. 



A single complete specimen, mentioned by Nelson as inde- 

 terminate, and a perfect left valve which appears in some 

 Zorritos material not exactly allocated, are referable to this 

 species. Slight differences, as noted above, exist between 

 the Peruvian and Antillean specimens, but careful com- 



