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1371 



TERTIARY FAUNA OF FLORIDA J/ 



Subgenus LUCINISCA Dall. 



Phacoides (Lucinisca) calhounensis n. sp. 

 PLATE 52, FIGURE 16. 



Oligocene of the silex beds at Ballast Point, Tampa Bay, Florida, and of 

 the Chipola beds at the Chipola River, Calhoun County, Florida ; Dall and 

 Burns. 



Shell resembling P. cribrarins Say in its general features, but with the sculp- 

 ture more dense, the reticulation finer and more even, the radial ribs being of 

 about the same strength and prominence as the concentric ridges, except near 

 the umbones ; the umbonal concentric ridges are less distant and prominent 

 and those close to the beaks are heavier and broader relatively to the size of the 

 shell, the anterior dorsal area is more conspicuous, the lunule proportionately 

 longer and larger, the crenulations of the inner margins of the valves finer and 

 more numerous. Alt. 10.0, Ion. 10.0, diam. 4.5 mm. 



This is in every way a smaller, finer, and more delicate shell than P. crib- 

 rarius. The silicious pseudomorphs from Ballast Point appear to have lost nearly 

 all their prominent sculpture and were probably consequent upon the fossiliza- 

 tion of worn, dead valves, which have a very different aspect from the perfect 

 shell. 



Phacoides (Lucinisca) plesiolophus Dall. 



PLATE 40, FIGURES 2, 5. 

 Lucina plesiolopha Dall, Trans. Wagner Inst, iii., p. 1196, pi. xl. figs. 2, 5, 1900. 



Upper Oligocene of the Oak Grove sands, Santa Rosa County, Florida ; 

 Burns. 



Shell thin, suborbicular, compressed, with high, pointed, prosogyrate beaks 

 and conspicuous dorsal areas ; concentric sculpture of low, rather strong, mod- 

 erately elevated ribs, tending to become obsolete towards the base and on the 

 anterior third of the disk ; they are also rather distant, more so than in any 

 of the other Tertiary species of this group ; radial sculpture of rather close-set, 

 rounded, low, even threads increasing by intercalation rather than divarication, 

 and stronger towards the ends of the valve ; the posterior dorsal area is 

 bounded in front by a rather strong radial rib ; on this the concentric ridges rise 

 into little, triangular leaflets, and similarly but less conspicuously on the pos- 

 terior dorsal margin ; there is a wider shallow sulcus on each side of this rib ; 

 the anterior dorsal area is very small and narrow with one or two radials upon 

 it ; the lunule is lanceolate, small, and emphatically excavated, larger in the left 



