103 SYSTEMATIC PALEONTOLOGY 



to 0.55 mm, in height, and with the thickness about the same as the 

 height. Valves moderately convex, the most prominent point being a 

 little in front of the postventral fourth, and just beneath the posterior 

 end of a single or double and more or less well defined longitudinal 

 depression. Left valve obliquely rounded in front, the outline here 

 being most produced in the lower half, and merging very gradually into 

 the long dorsal edge, posterior margin also oblique, though less so and 

 with the greatest prominence in the upper half ; dorsal and ventral mar- 

 gins nearly straight and parallel,, the slight curvature in both being 

 upward; posteriorly a wide and rather sharply defined flattened border, 

 narrowing ventrally, and a rather obscurely defined one anteriorly. Eight 

 valve smaller than the left and quite different in outline, the latter 

 being due to two excisions at the extremities of the hinge, the anterior 

 one of which is slight, the posterior one deep. Hingement strong, 

 normal for the genus. Surface with scattered pits, sometimes obscure, 

 often restricted to the broad, subcentral, longitudinal furrow or 

 furrows. 



We know of no described species with which this need be closely 

 compared. It is probably nearest our C. plebeia, but the median furrow 

 and the well developed posterior border of the left valve are both lack- 

 ing in that species. 



Occurrence. — Calvert Formation. Plum Point. 



Collections. — U. S. jSTational Museum, Maryland Geological Survey. 



Cythere plebeia n. sp. 

 Plate XXXV, Figs. 20-29. 

 Desc7-iption. — The carapace of this species closely simulates that of 

 C. incequivalvis in outline of both valves, the differences between them 

 being chiefly in their surface markings. Thus, the valves of C. plebeia 

 are more evenly convex and show no sign of the broad, subcentral 

 longitudinal furrows characterizing C. incequivalvis nor of the flattened 

 border that is developed, particularly along the posterior edge of the 

 left valve, in that species. The surface pits also are smaller and less 

 irregular, and generally the pits exhibit the peculiarity of occurring in 

 pairs. Comparing outlines of the left valves of the two species, it will 



