ism, and that genus also may be weakly sulcate ; 

 the sulcus in Cyprideis, however, is more medially 

 located and generally broader and shallower than 

 in Fossocytheridea. The new genus, like Cypri- 

 deis, probably represents lagoonal or brackish- 

 water environments. 



Fossocytheridea lenoirensis Swain and Brown, 

 n. sp. 



Plate 1, Figures lla-h; Plate 2, Figures la-j, 2 

 Text Figures 5a, b 



Shell of mature female subovate in side view, 

 highest medially ; dorsal margin of left valve 

 strongly convex, umbonate, with somewhat 

 steeper anterior than posterior slope; dorsal mar- 

 gin of right valve moderately convex, truncate 

 along its posterior slope. Ventral margin of left 

 valve nearly straight, curving more abruptly into 

 anterior than into posterior margin ; ventral mar- 

 gin of right valve concave, slightly sinuous an- 

 teriorly. Anterior margin of left valve broadly 

 rounded, slightly extended below; posterior mar- 

 gin somewhat more broadly rounded and slightly 

 extended medially. Valves strongly convex, with 

 greatest convexity occurring about one-third from 

 posterior end. Left valve larger than and extend- 

 ing strongly beyond right, except posteromedially. 



Surface of right valve bears an anterodorsal 

 narrow, fairly deep sulcus that extends ventrally 

 from a position near dorsal margin, about one- 

 third from anterior end, to a little dorsal to mid- 

 height where it gradually dies out. Some mature 

 specimens have low broad nodes in front of and 

 behind sulcus and an additional one or two faint 

 nodes in posterior third of valve. Surface, pos- 

 terior to sulcus, markedly swollen. Corresponding 

 sulcus on left valve is shorter and more pitlike, 

 and nodes are less commonly present. Posterior 

 portion of right valve slightly compressed and its 

 margin bears a short median flangelike extension 

 that results in little or no posterior overreach of 

 right valve by left. General surface in well pre- 

 served specimens marked by rather large, widely 

 spaced shallow pits. 



Hinge of left valve composed of a slightly an- 

 teromedian, elongate, socket depression contain- 

 ing about eight cross-ridges, an interterminal high 

 bar formed of the fusion of two shell laminae that 

 in turn separate to define the terminal sockets; 

 crest of bar has a very shallow, weakly crenulate 

 groove, and a posterior shorter, denticulate de- 

 pression containing six to eight cross-ridges. 

 Hinge of right valve has terminal elevated denti- 



culate areas, in the posterior of which the cross- 

 ridges have double crenulations or, expressed 

 differently, the cross-grooves are alternately shal- 

 lower and deeper; and an interterminal crenulate 

 depressed portion. Muscle scar an anteromedian 

 vertical row of four spots with an additional two 

 spots lying in front of upper and lower ends of 

 main row; inner lamellae narrow, broadest ante- 

 riorly ; only a narrow vestibule terminally. Radial 

 canals number about 25 anteriorly and tend to oc- 

 cur in pairs, one above the other; some of canals 

 have median expansions ; canals are fewer in num- 

 ber posteriorly and scarce ventrally. Normal 

 canals vary from fairly numerous to sparse ; they 

 apparently do not necessarily correspond in posi- 

 tion to surface pits. 



Shell of mature male subelliptical in side view, 

 left valve much less umbonate than in female and 

 right valve is somewhat more elongate than in 

 corresponding female shell. Convexity of male 

 shell much less than that of female. Other shell 

 features of the dimorphs are the same. 



Immature molts are characterized by more tri- 

 angular outline, weaker dentition and other mar- 

 ginal features and more prominent nodes on sur- 

 face of valves. 



Length of holotype, a female left valve, 0.88 

 mm, height 0.56 mm, convexity of valve 0.30 mm; 

 length of paratype male shell 0.82 mm, height 

 0.46 mm, convexity 0.36 mm. 



Relationships. — The new species somewhat re- 

 sembles Cyprideis littoralis Brady (1869) in gen- 

 eral shape, convexity and presence of a sulcus. 

 The sulcus in C. littoralis however is more nearly 

 median in position and is broader and shallower 

 than in F. lenoirensis. The latter also is much 

 more umbonate in the left valve than C. littoralis. 



Occurrence. — Rare to abundant in beds of prob- 

 able Washita (Albian) age, subsurface of north- 

 eastern North Carolina. It has been found in the 

 following wells; DuPont water well No. 6, Lenoir 

 County, 723 feet and below ; Greenville city water 

 well, Pitt County, 608 feet and below; Caledonia 

 Prison Farm water well No. T-2, Halifax County, 

 271 feet and below; Caledonia Prison Farm water 

 well No. T-4, 141 feet and below; Scotland Neck 

 water well No. T-l, Halifax County, 271 feet and 

 below; Scotland Neck water well No. T-2, 270 

 feet and below; Scotland Neck water well No. T-3, 

 278.6 feet and below; Odom water well No. T-l, 

 Northampton County, 108 feet and below; Odom 

 water well No. T-3, 185 feet and below. 



20 



