339 Costa Rica Miocknk — Oi^sson 167 



smaller intermediate ones in the interspaces; the ribs become 

 gradually obsolete with age and the later portions of full-grown 

 shells are nearly smooth; fine transv^erse lines of growth cross the 

 surface and are best seen on the earlier portions of the shell ; 

 operture oblique. 



Length 60, diameter 12 mm. 



This large Dentaliuni is abundant and very characteristic of 

 certain conglomeritic zones in the Upper Uscari shales. There it 

 occurs associated with a small but very distinctive fauna, includ- 

 ing the Ptychosalpinx ? deiitalis, Sco7isia cocleana and Scaphella 

 costaricana and other species. When complete, the D entalhuii 

 uscaria7i7im frequently reached a length of no mm or more. 



All our specimens are unfortunately lacking in the apical 

 tip. In general sculpture, it recalls the Dentaliiim floridense 

 Henderson dredged from 35 to no fathoms of w^ater off Florida 

 and in the West Indies, but the taper of the shell is more gradual 

 and the sculpture more irregular, becoming obsolete with age. 

 Bose* has described a large species as Dentalium rimosuni from 

 the Miocene of the Teliuantepec Peninsula. In that species, the 

 ribs number about3i, are very regular with deep interspaces, and 

 ribs are finely etched by transverse lines of growth. 



Uscari Stage: Coco Plum^ Panama. 

 Rio Codes. 



Margarila — Old Harbcr trail. 

 Comadre Creek. 

 Pumbri Creek, etc. 



*Boll. Inst. Geol. do Mexico, No. 22, p. 55, pi. 3, figure i, 1906. 



