FROM PLIOCENE CLAY BEDS IN COSTA RICA. 375 



Length 1.5 inch; width 1.25 inch. 



This species is of the style of C. subeJongatum, Sby., but the ribs are not so 

 prominent and the upper edges of the valves are not so serrate, and are more regu- 

 larly sloping from the hinge on both sides than in any other species. 



C. (fragum) callopleurum, n. s., PL 47, fig. T7. 



Shell suboval, oblique, convex; beaks placed in advance of the median line; 

 posterior side produced above, convexly truncated; posterior basal portion pro- 

 duced, rounded; surface marked by eighteen to twenty prominent ribs, about six 

 of which, on the middle of the shell, are the largest. These ribs are subacute on 

 top, with sloping sides, and each bears a series of closely placed, bead-like tuber- 

 cles on its upper edge; the interstices between the ribs are crossed by strong irre- 

 gular lines. Inner surface marked by deep grooves corresponding to the external 

 ribs. 



Length 0.9 inch; width 0.75 inch. 



C. Haitense, Sby., a common fossil of Santo Domingo, and recently reported as 

 living in the West Indies,* is the nearest allied species. The differences are: 

 That shell has from twenty to twenty-four ribs ; while the maximum number of 

 this is twenty. In that the ribs are square on top j in this they are subacute ; the 

 tubercles in Haitense are much larger than in the present species; the internal 

 surface of the callopleurum is deeply grooved, while in Haitense no traces of the 

 external ribs extend beyond the pallial line; and finally, while C. Haitense varies 

 considerably in the strength of the umbonal angle, some specimens falling off 

 from a prominent rib at a right angle, while others are quite rounded; in the pre- 

 sent species there is no trace of such an angle, the surface curving over gently 

 from the posterior area to the top of the shell. 



PAPYRIDEA, Sw. 

 P. BULLATA, L. (Sp.), Enc. Meth., PI. 296, f. 6. 

 Abundant. Living in the West Indies. 



CHAMA, Linn. 

 C. MACROPHYLLA, Chemn. Conch., v. 7, PI. 52, figs. 514, 515. 



Common. Living in the West Indies. i 



* Guppy, Aun. Mag. N. Hist., 4 sen, v. 15, p. 51. 



