395 



Costa Rica Miocene— Olsson 



223 



Length 11.50, height 11.00, diameter of the left valve 

 2.25 mm. 



This species is recognized by its depressed or but slightly 

 convex shell and coarse sculpture, of which the radial is a little 

 stronger. Phacoides cribraritts Say and nassula Conrad are both 

 more convex, with the concentric lamellae more distantly spaced 

 on the umbos and dominating over the radial. P. muricatus 

 Spengler of the West Indies is much more finel}^ sculptured. 



We have also a young shell from Water Cay which is pos- 

 sibly P. nassula^ var. caloosana Dall of the Florida Pliocene. It 

 is very close to specimens of that species in the Cornell collec- 

 tion. 



Gatu7i Stage: Middle a^eek. 



Phacoides bocasensis, n. sp. Plate 32, figure 6 



Shell thin, subcircular, depressed; dorsal area well defined 

 by a marked change in sculpture; beak small, but distinct, placed 

 a little in front of the middle; the sculpture on the disk of the 

 shell is fine, composed of subequal, radial threads, separated by 

 narrow interspaces; the ribs and interspaces are crossed by fine, 

 concentric threads or ridges which produce a finely scabrous 

 surface; the concentric threads are a little more widely spaced 

 on the umbos; the dorsal area is defined by a stronger radial, be- 

 yond which follows a wide radial band, strongly sculptured with 

 the concentric threads or lamellse but lacking in radials; the up- 

 per half of the dorsal band bears in addition to the concentric 

 lamellae, 2 or 3 weak but scabrous radial threads; the lunule is 

 very narrow, lanceolate and with a few strong, concentric lame- 

 llse; interior of the shell cavit}^ shallow, with the ventral margin 

 finely crenulated. 



Length 10.00, height 9.25, diameter of the left valve 

 2.00 mm. 



A depressed and very finely sculptured species whose gener- 

 al form is that of P. muricatus Spengler of the recent West In- 

 dian fauna. It is more regularly sculptured than the P. hispan- 



