335 Costa Rico Miocene — Olsson 163 



GeniisCHLOROSTOIVlA Swainson 



Subgenus NEOlViPHAL! US Fischer 



Chforostoma costaricensis, n. sp. Plate 15, figure 16 



Shell conoidal, solid, with a narrow, but deep umbilicus; su- 

 ture distinct; the last whorlis strongly carinate or angled below; 

 above this angle, the whorl is evenh' convex, below, the base is 

 nearh' fiat; the sculpture consists of beaded spiral cords, there 

 being on the last whorl above the angled periphery 7 or 8 cords; 

 the interspaces are nearl}^ as wide as the cords, and are finely 

 etched by oblique lines of growth and finer spirals; on the base, 

 the spirals are irregular, the strongest about the middle; there 

 are 2 or 3 cords about the upper part of the umbilicus, but the 

 interior wall is smooth. 



Height 10.5, diameter 16.5 mm. 



The transitional beds between the Uscari and the Gatun of 

 Red Cliff Creek, contain in addition to the usual fauna, several 

 species, as certain Cerithoids, Sistrum nodulosum, Modulus mo- 

 dulus, Nitidella and the present species, whose normal habitat 

 is on the rocks and reefs of the extreme, upper limit of the lit- 

 toral zone. They are always much broken and worn as if the}' 

 had been rolled and washed about by the surf on a rocky or sandy 

 beach and then drifted out to the deeper and quiter waters off 

 shore where depostion was going on. 



Although all of our specimens of C. costaricensis are very 

 fragmentary and much worn, we have deemed it best to place 

 the species on record, as this subgenus has been recorded only 

 very doubtefully from beds older than the Pleistocene. 



Uscari Stage: Zone 6, Red Cliff Creek. 



Gatun Stage: Zone 5, Red Cliff Creek. 



