Bulletin 39 



3«2 



to the Pliocene V\'here its acme of development was reached in 

 the elegant T. preattenuaia Heilprin of Florida. It has left no 

 descendants in the recent fauna. 



The shell is long and tapering, each whorl sculptured with 2 

 prominent beaded spiral cords, between which the surface is con- 

 cave or deeply channeled and may be smooth or with finer 

 spiral threads. The upper spiral cord is simple or double. Several 

 varieties ma}- be recognized which differ in details of their sculp- 

 ture, size and form of shell. 



T. alii lira Conrad, typical Plate 14 figures 6, 7 



This, the most elegant form is limited to the small Gatun 

 sedimentary basin of the Canal Zone and the adjacent Province 

 of Colon. 



The shell is long, slender and with numerous whorls; each 

 whorl is strongly sculptured by the two, primary spiral cords, 

 each of which is bordered on adjacent faces by a smaller 

 spiral, between which lies a deeply channelled groove; small- 

 er beaded spirals occur in the median channel and are best devel- 

 oped on the spire whorls; the beading or granulation of the 

 spirals is produced by the arcuated lines of growth, the sinus of 

 which lies a little above the middle of the shell; the upper spiral 

 cord is larger than the lower and more strongly beaded and in 

 general becomes double on the later whorls. 



Gatun Stage: Canal Zone and Province of Colon. 



Var. chiriquiensis, n. var. Plate 14, figures 4, 8, 9, 14 



Shell larger, less attenuated and more coarsely sculptured 

 than typical altilira\ the 2 spiral cords are less strong, of more 

 nearly the same size and usually remaining simple; the 2 spiral 

 cords enclose a wide, concave area of about the width of the 

 whorl; this concave band usually carries 2, 3, or more fine spiral 

 threads; the beading or granulation of the spiral cords is feeble 

 and best developed on the earlier spire whorls and in certain large, 



