MUSEUM OF COMPARATIVE ZOOLOGY. 21 



Shell uniform light brownish-white; with twelve principal radiating ribs 

 on each A'alve, and secondary riblets between them, toward the margin. 

 Neural valve with a more or less marked depression extending from the 

 beak to the anterior margin, where it forms a slight convexity. Corre- 

 sponding to this internally is a slight rounded ridge. Hinge line straight, 

 sides and anterior margin slightly rounded. Area flat, smooth, as wide as 

 the shell. Pscudo-deltidia large, triangular, widely separated. Foramen 

 very large and usually much eroded. Hinge teeth moderately large and 

 strong. Muscular impressions very posterior, hidden beneath the area 

 when viewed from above. Margin slightly crenulated. A few faint stria? 

 discernible upon the surface of the ribs. Haemal valve smaller, flatter, 

 with a straight hinge line slightly emarginated in the middle, no area or 

 cardinal process ; teeth and sockets large and strong. Septum large and 

 stout, composed of three or four radiating ribs, with thin shelly matter be- 

 tween them, forming nodules and notches on the upper edge ; the whole 

 of a subtriangular form somewhat resembling a half-opened fan. Pos- 

 terior edge slightly concave, reaching a little behind the middle of the 

 shell ; anterior edge reaching the anterior margin of the shell, which 

 is here slightly concave or emarginated, giving the valve a somewhat 

 bilobed appearance. Muscular impressions much thickened, forming 

 two rather concave disks. Apophyses consisting of two haemal bands 

 attached to the hinge margin, first with two broad crura pointing 

 toward the median line, the arms of the apophyses extending in a 

 rounded curve within the middle third of the shell, and attached by 

 their lower edges to the thick disk-like muscular scars, and, lastly, to 

 the septum on each side about its middle, close to the shell. Cardinal 

 plate, or hinge plate, absent. Area behind the muscular disks somewhat 

 excavated. 



The anterior portion of the apophyses is more posterior than in C. 

 Neapolitana, and the margin is not granulated as in that species. It would 

 seem from Mr. Davidson's figures that the loop of Cistclla Barretliana Dav., 

 is more anterior than in this species ; the latter being also unprovided 

 with the posterior extension of the septum seen in the figures of the for- 

 mer, and being, moreover, entirely destitute of the red markings between 

 the ribs. It agrees with C. Antillarum Crosse and Fischer, as figured in gen- 

 eral appearance, but wants the red markings attributed, in the description, 

 to that species, and the ribs are also carried over the apex, while that por- 

 tion of C. Antillarum is described as smooth. No comparisons can be drawn 

 in regard to the apophyses, as Crosse and Fischer did not figure those of 

 their species. It is possible that the present species, C. Antillarum and C. 

 Barretliana are forms of one species, in which case the last name has pri- 

 ority, or it may be that the two latter are distinct from the present species, 



