AitT. 3 5 MOLLUSKS OF THE GENUS CORBICULA MARSHALL. 3 



form. The young have an oblique splash of purple in front and 

 back of the beaks, both splashes pointing in a general way toward 

 the posterior ventral angle. The beaks of the mother shell are much 

 eroded so that purple as splashes can not be seen, but the beak in 

 general is purple. 



Data regarding the nepionic young of the various species are woe- 

 fully lacking, but from what has been observed one gathers the idea 

 that a very careful study of the embryology of the Corhiculas would 

 lead to a more accurate understanding of what species there are. 

 This, with geographic distribution, should clarify our knowledge of 

 the South American species of this genus. 



CORBICULA (CYANOCYCLAS) CIRCULARIS, new species. 



Plate 2, figs. 1-3. 



Shell subcircular in outline, very compressed, rather solid, poste- 

 rior margin very slightly, obliquely truncated. Hinge line much 

 arched, anterior and ventral margins regularly, nearly circularly, 

 curved ; beaks not much elevated, posterior and anterior ridges nearly 

 equal in height, the whole surface being nearly of a uniform con- 

 vexity ; color in type bleached straw, becoming greenish around the 

 margin (in the cotype the whole surface is a dirty greenish with 

 an undertone of straw color) ; sculpture consisting of a number of 

 engraved concentric lines with concentric striae between them and 

 several more plainly marked growth lines. Both cotjq^es are left 

 valves. The hinge area broad, the middle cardinal tooth bifid, the 

 anterior one smaller but strong, the posterior one very small and 

 weak, long and knifelike. Anterior lateral standing out prominently 

 on the hinge plate, thin, long, slightly bowed, undulated on its edge, 

 coarsely striated and with radiating oblique grooves on its outer 

 surface. Posterior lateral holding the same direction as the third 

 cardinal, slightly bowed, its front end elevated, its edge crenulated, 

 and its outer face rudely striated. The ligamental scar (sinulus) 

 relatively large. Pallial line in type about 5 mm. from ventral mar- 

 gin, well marked, the sinus rather small. Color of interior of both 

 cotypes plain white. 



The type. Cat. No. 347860, U.S.N.M., measures: Length, 25 mm.; 

 height, 24 mm. ; diameter if both valves were present would be 12 mm. 

 The cotype measures : Length, 20 mm. ; height, 18.5 mm. They come 

 from the Uruguay River, LTruguay, and were presented by Doctor 

 Felippone. 



The nearly circular outline, the compressed form, and the nearly 

 regular convexity distinguish this species from all others yet known. 



