shape and spurlike extensions of the upper of the 

 two ventral ridges, together with the median and 

 dorsal longitudinal ridges. 



Length of figured specimen 0.65 mm, height 

 0.33 mm, convexity 0.28 mm. 



Remarks. — The generic status of this species 

 remains uncertain, but the hinge described by 

 Bolin (1956, p. 296) is like that in Cythereis. The 

 species belongs in the general group of C. worth- 

 ensis (Alexander) (1929, p. 92), C? gapensis 

 (Alexander) (1929, p. 84) and C. pidgeoni (Ber- 

 ry) (1925, p. 425) which have narrow longitu- 

 dinal ridges from which short spurs extend per- 

 pendicularly. 



Occurrence. — The species is rare in the Humble 

 Oil and Refining Company's Tindel No. 1 well, 

 Jackson County, Florida, core sample 2,881-2,891 

 feet, in beds of Eagle Ford age (upper member 

 of Atkinson Formation). It was described from 

 beds of Eutaw (?) and Black Creek (?) age in 

 wells in the Cape Hatteras area, North Carolina 

 and has been recorded from the Upper Creta- 

 ceous of Minnesota. 



Cythereis sp. 



Plate 3, Figure 10 



A few poorly preserved specimens illustrated 

 herein are similar to C. qimdrialira (Swain, 1952, 

 p. 84) but the anterior margin is blunter, trun- 

 cate above and the posterior margin is less pointed 

 than in C. quadtialira; the surface ornamentation 

 is similar to that species. 



Length of figured specimen 0.57 mm, height 

 0.28 mm, convexity 0.23 mm. 



Occurrence. — Rare in lower member of Atkin- 

 son Formation, Coastal Petroleum Company's No. 

 1 Wright well, Pinellas County, Florida, core sam- 

 ple 7,100-7,197 feet. 



Cythereis f redericksburgoides Swain and Brown, 

 n. sp. 



Plate 3, Figures 9a-f; Text Figure 10a 



Shell subquadrate-subacuminate in side view, 

 highest about one-fifth from anterior end; hinge 

 margin somewhat sinuous but fundamentally 

 straight, with strong projection at position of 

 greatest height; both cardinal marginal bends 

 are obtuse, the anterior less broadly so than pos- 

 terior; ventral margin nearly straight, converg- 

 ing slightly with dorsum in a posterior direction ; 

 anterior margin broadly rounded, slightly extend- 



ed below, subtruncate above, bearing a few short 

 spines ; posterior margin subacuminate, strongly 

 extended ventral to midheight, truncate to some- 

 what concave above, with several short marginal 

 and submarginal spines. Left valve slightly larger 

 than right, extending beyond edge of right antero- 

 dorsally and along posterodorsal slope. Terminal 

 parts of valves strongly compressed, ventral sur- 

 face flattened due to alate expansions ; midportion 

 of valves not strongly convex, greatest expansion 

 posteromedian in position. 



Anterior marginal area bears a narrow-crested 

 rim from both flanks of which extend five or six 

 narrow perpendicular ridges ; marginal rim ter- 

 minates dorsally in eye tubercle at position of 

 greatest height, and ventrally rim merges with 

 surface at similar position ; posterior margin also 

 provided with a narrow low rim along ventral sur- 

 face of which occur submarginal spines. Along 

 midventral three-fifths of valve is a submarginal 

 rather broad alate ridge that attains greatest 

 height posteriorly and provides flattened wedge- 

 shaped ventral surface of shell; dorsal margin 

 also bears a narrow, low, in part spinose or nodose 

 rim, which extends from a short oblique sulcus in 

 front of eye tubercle to about one-fourth from 

 posterior and where it terminates in a pointed 

 elevation, but also bends ventrally a slight 

 amount. Anteromedially on each valve is a large 

 elongate swelling, posterior to which is a weak 

 broad ridgelike swelling. General surface of valves 

 both coarsely reticulate and finely pitted ; in some 

 specimens one or the other predominates. 



Hinge of left valve, and anterior rounded deep 

 pit and posteriorly adjacent high curved tooth, an 

 interterminal bar and a posterior slightly oblique 

 deep socket. Inner lamellae moderately wide ; line 

 of concrescence and inner margin slightly sepa- 

 rated anteriorly. 



Length of holotype shell 0.73 mm, height 0.37 

 mm, convexity 0.36 mm. 



Relationships. — Cythereis fredericksburgensis 

 Alexander (1929, p. 89) from the Goodland Lime- 

 stone of Texas is similar to the present form in 

 shape and main features of surface ornamenta- 

 tion. That species however lacks the terminal or- 

 namentation of the present one and has a more 

 continuous median longitudinal ridge. 



Occurrence. — The species occurs in the lower 

 member of Atkinson Formation in the following 

 wells in Florida and Georgia: (1) Sun Oil Com- 

 pany's Westbury No. 1 well, Putnam County, 



34 



