333 Costa Rica Miocene— Olsson 151 



gerontic shells the spiral cords may become nearly smooth. 



This is the common form on the islands of the Chiriqui La- 

 goon, and of eastern Costa Rica. Like the t3'pical altilira it is 

 often a very abundant fossil, forming zones of Ttirritella in the 

 Gatun beds. 



The sculpture is much less elegant than that of true altilira, 

 and consists mainly of the 2, primary spiral cords, between which 

 lies a wide concave band. This band is but feebly sculptured by 

 small, spiral threads, or in some cases, is smooth. 



In small matters of detail of sculpture and in the form and 

 taper of the shells, there is considerable variation. Such varia- 

 tion is usually of but local significance and in general the shells 

 collected at the same or nearby localities will agree closly with 

 each other but differ more or less from those of more distant 

 areas. 



The T. tornata Guppy from the Miocene of Venezuela and 

 Trinidad should probably be considered as a variety of altilira, 

 most closely allied to the chiriquensis . Its sculpture is like chir- 

 iquensis but with stronger spiral threads on its median band. 



Gatun Stage: Water Cay. 



Grape Point and Red Cliff Creeks, C. R. 

 Banayia River. 

 Rio Blanco. 



Var. COStaricensis, n. var. Plate 14, figure 11 



Shell small; whorls less numerous and more rapidly increas- 

 ing so that the shell is porportionately broader and shorter; su- 

 tures deep; the 2 primary cords are but weakly developed, be- 

 tween which lies, the concave band as in chiriquiensis; the band 

 carries 3, 4, or more strong, irregular spirals, but slightly weak- 

 er than the primaries; the granulation of the cords is weak and 

 irregular. 



Much smaller than the preceding varieties (usually about 40 

 mm) and with wider and more rapidly increasing whorls. The 



