LAND AND FRESH-WATER FAUNA OF THE SILEX-BBDS, 



This peculiar association of species whose abundance marks a special 

 phase in the history of the silex-beds, namely, the culmination of the upward 

 movement, I have decided to treat separately. This fauna was not of sudden 

 appearance. Doubtless as soon as small hummocks became sufficiently eleva- 

 ted to be above the incursions of the tide they were invaded by the operculate 

 Helicina and the beach- frequenting Stropliia. The other land snails were 

 probably later, but the fresh-water species would necessarily be delayed until the 

 supermarine area was large enougkto afford fresh-water ponds or streams not 

 invaded by salt water. These last were naturally the latest and are the rarest, 

 while the Helicina is found more or less mixed with the marine shells lower 

 down, where it had been blown by the winds or washed by extraordinary 

 storm-tides. 



Family VIVIPARID^.' 



Genus LIOPLAX Troschel. 



Lioplax floridana n. s. 



Plate I, figure 3. 



Shell thin, smooth, or slightly marked by incremental lines ; the upper or 



posterior half of the whorls somewhat flattened ; the periphery obtusely 



keeled; base well rounded; umbilicus none, whorls probably four or five, the 



type specimen decollated ; suture distinct but not channelled ; aperture 



rounded in front, narrow but not acute behind; outer lip, simple, thin. Lon. 



of decollate specimen 7.5 ; lat. of do. 6.0 mm. 



A single specimen in poor condition, but characteristic, was collected by 

 the writer at Ballast Point. It is sufficiently distinct from the L. siibcarinata 

 Say to need little comment; the flattish and peripherally angular whorls of 

 the fossil enable one to discriminate between them at once. 



Family HELICINIDyE. 



Genus HELICINA Lamarck. 



Helicina ballista n. s. 



Plate I, figures 2, 2 a. 



Shell depressed-conic, five-whorled, solid, spire less than half the "height 

 of the shell ; whorls slightly flattened above ; suture distinct, not deep ; sculp- 

 ture of incremental lines and occasional not prominent wrinkles, crossed by 

 faint impressed lines, with subequal interspaces, to the number of six or seven 

 on the last whorl above the periphery ; base full, evenly rounded, with a thin, 

 smooth central callus ; aperture oblique, semilunar ; lip reflected, and in old 

 specimens very thick, narrowest and thinnest near the pillar, which is very 

 short and almost unrecognizable as distinct from the lip ; alt. of shell 7.0 ; 

 max. diam. lo.o; min. diam. 8.0 mm. 



