ventrally, and little or not at all terminally. Valves 

 moderately convex, with greatest tumidity mid- 

 ventral in position. Surface smooth, except for a 

 weak dorsomedian sulcus on right valve. 



Hinge of left valve consists of an anterior short 

 pitlike socket that is traversed by one or two low 

 ridges, an interterminal strongly and deeply den- 

 ticulate taxodont zone, broadest anteriorly and a 

 posterior elongate, finely denticulate shallow sock- 

 et. Hinge of right valve correspondingly bears an 

 anterior short notched tooth, an interterminal 

 denticulate zone and a posterior crenulate tooth- 

 like low elevation. 



Muscle scar is composed of a distinctly antero- 

 median row of four small closely spaced spots 

 plus a curved or double spot lying well in front 

 of upper end of main group. Inner lamellae fairly 

 narrow, broadest anteromedially, where a small 

 vestibule occurs; otherwise line of concrescence 

 and inner margin nearly or entirely coincide. Ra- 

 dial canals few and widely spaced, numbering 

 about six at each end. Anteromidventral margin 

 of each valve bears a prominent selvage apophy- 

 sis. A low dorsomedian ridge on interior surface 

 of each valve represents position of exterior weak 

 sulcus on right valve, sulcus barely discernible or 

 absent on exterior of left valve. 



Length of holotype shell 0.51 mm, height 0.32 

 mm, convexity 0.22 mm. 



Relationships. — This species resembles E. semi- 

 constrictum (Alexander) (1933, p. 198) from the 

 Kiamichi Formation (Albian) of Texas but that 

 species is less umbonate dorsally. E. paenorbicula- 

 tum (Alexander) (1933, p. 197) from the Good- 

 land Limestone (Albian) of Texas is strongly um- 

 bonate but its caudal process is more ventral in 

 position and is not upturned. 



Occurrence. — Early Cretaceous, probably of Al- 

 bian or upper Aptian age, equivalent to the mid- 

 dle or upper parts of Fredericksburg or lower 

 part of Washita Groups; frequent in Greenville 

 City well, Pitt County, North Carolina, 608-685 

 feet. 



Eocytheropteron venoides Swain and Brown, 

 n. sp. 



Plate 3, Figures 12a-f; Text Figure 11a 



Shell of presumed male dimorphs small, short- 

 ened-subpyriform in side view, highest medially 

 to anteromedially ; dorsal margin strongly convex, 



angulated at position of greatest height, posterior- 

 dorsal slope somewhat truncate; ventral margin 

 moderately convex, slightly sinuous posteromedi- 

 ally; anterior margin narrowly rounded, marked- 

 ly extended ventral to midheight; posterior mar- 

 gin narrowly curved, with short bluntly pointed, 

 slightly upturned caudal process at midheight. 

 Left valve larger than right, overlapping and ex- 

 tending beyond right along all but posterior mar- 

 gin. Valves strongly convex, greatest tumidity lies 

 posterior to and slightly ventral to middle of shell. 



Surface ornamentation varies from nearly 

 smooth to weakly ornamented with narrow closely 

 spaced ridges which are longitudinally arranged 

 and converge backwards in posterior half of shell, 

 and are irregularly arranged anteriorly. 



Hinge of left valve composed of an anterior 

 elongate taxodont socket with about five denti- 

 cles, an interterminal rather short crenulate 

 groove, and a posterior crenulate elongate socket, 

 also with about five denticles. Muscle scar con- 

 sists of an anteromedian curved row of four spots 

 and an additional horseshoe-shaped spot in front 

 of upper end of main group. Inner lamellae rather 

 narrow, line of concrescence slightly separated 

 from main margin anteriorly. Radial canals few 

 and widely spaced; normal canals not observed. 

 Anteromidventral margin of left valve bears a 

 prominent selvage apophysis which fits over a sim- 

 ilar structure of right valve. 



Shells of presumed female dimorphs are more 

 elongate and have the caudal process less sharply 

 defined. 



Length of holotype shell 0.45 mm, height 0.27 

 mm, convexity 0.26 mm. 



Length of a presumed female left valve 0.55 

 mm, height 0.30 mm, convexity of valve 0.17 mm. 



Relationships. — Eocytheropteron bilobatum 

 (Alexander) (1929, p. 104) of the Weno forma- 

 tion (Albian) of Texas is similar to E. venoides, 

 n. sp. in shape, overlap relationships of the valves, 

 and character of the caudal process. Alexander's 

 species however, is weakly sulcate dorsomedially, 

 and the surface is finely punctate rather than with 

 venose ridges as in the new species. The dimorph- 

 ism of the shells of this species is unusual among 

 described representatives of the genus. 



Occurrence. — Several specimens of the species 

 were found in the Early Cretaceous, probably of 

 Washita (Albian) age, in the Greenville City well, 

 Pitt County, North Carolina in a composite sam- 

 ple from 608-754 feet. 



38 



