LOWER HELDERBERG ROCKS. 373 



Leperditiu alta. 



Plate LXXIX A. Fio. 6 a - «. 

 Cytherina alta : Conkad. VANUXEit, Geological Report of the Fourth District, 1843, p. 112, f. 6. 



Carapace subreniform : valves strongly convex, gibbous in the middle, 

 somewhat variable in their proportions, straight above and broadly 

 rounded below, the extremities obliquely rounded, the dorsal angles 

 varying in their degree of development ; posterior side broader than 

 the anterior : ventral margin of the right valve abruptly incurved, 

 sometimes leaving an apparently obtuse carination along the line of 

 bending. Anterior or eye tubercle distinct in the test, and sometimes 

 distinct in the casts : not unfrequently there is a little inequality of 

 surface between the anterior tubercle and the anterior dorsal angle. 

 Central tubercle a gibbous area, not defined on the exterior surface, but 

 from which, in casts and partial casts, the internal vessels are seen to 

 radiate. 



The anterior edge often presents a slight marginal rim, but this is rarely 

 Been on the posterior border of the specimens before me. The ventral 

 edge of the right valve is abruptly incurved over the edge of the left 

 valve, and its inner margin shows two or three fine concentric lines 

 and numerous fine crenulations ; while the inner edge of the dorsal 

 margin shows similar crenulations, which are perhaps a little stronger 

 than those on the ventral side. The ventral margin of the left valve is 

 bent inwards almost rectangularly to the axis ; the included surface, 

 or that covered by the right valve, limited by a thin sharp slightly 

 elevated ridge ; while on the anterior and posterior margins, the space 

 between this ridge and the inner edge of the valve is contracted to a 

 narrow well-defined groove. In older shells, the inner ventral edge of 

 the right valve presents a strong groove for the reception of the ventral 

 edge of the left valve. 



Surface essentially smooth, or so finely granulose as not to be visible 



