379 



Costa Rica Miocene— Olsson 



207 



narrow and appears only with maturity and is more or less hid- 

 den by the strongly incurved beak; the sculpture consists of 

 vStrong radial cords and small ribs divisible into 3 sets; a primary 

 set of 7, which in perfect specimens and in the 3^oung carry large, 

 strong spines; a secondary thread in each of the wide interspaces 

 and when perfect bearing smaller spines, and tertiar}^ threads of 

 a variable number between the secondaries and primaries; the 

 whole surface is in addition, finely, longitudinally starited with 

 minute, scale-bearing threads; j'oung shells have a small foliace- 

 ous area on the umbo of the sessile or right valve and strong 

 spines on both valves. 



Height 151, length 120, diameter 115 mm. 

 A large, ponderous species with nearly equal and strongly 

 convex valves which show no distortion due to attachment 

 Young shells associated with the type have a small, somewhat 

 foliaceous attachment area on the right valve, and each valve 

 bears large, strong spines on its primary set of radial cords and 

 finer spines on its secondary and even on some of the tertiary 

 threads. 



Of described species, the chiriquiensis seems nearest related 

 to the bostrychites Guppy from the Miocene of Santo Domingo 

 and Jamaica but differs by its much larger size, proportionately 

 more convex whorls and by its cardinal area which is more up- 

 right and with a more incurved beak at its upper end. The 

 sculpture of chiriquicjisis is more spin3^,the spines being borne by 

 the primary radials and to a less degree by the secondaries, and 

 even by the tertiaries. In bostrychites only the primaries bear 

 spines, the others appearing as large, nearly smooth, intermedi- 

 ate threads and cords. We have specimens of a large, closely 

 allied Spondylus from the Chesapeake Miocene of Florida, dif- 

 fering mainly in its cardinal area and in details of sculpture. 



Gafun Stage: Water Cay. 



Spondylus gumanomocon Brown and Pilsbry Plate 21 figure i 



Spondylus americanus Gabb, 1873, Trans. Amer. Phil. Soc. , vol. 15, 

 p. 257. Not of Ivamarck. 



