106 SYSTEMATIC PALEONTOLOGY 



valve, barely distinguishable. A wide, smooth, depressed border 

 encloses the ends, grows narrow along the dorsal edge and appears to 

 die out entirely, in a lateral view, along the ventral edge. Eemainder 

 of surface moderately tumid and marked by rather large pits arranged 

 in lines, those in the posterior and central parts longitudinal, those 

 along the ventral and anterior sides irregularly concentric and those 

 on the dorsal slope somewhat obscurely radial. Just above and in front 

 of the center of the valves there is sometimes a slight swelling which 

 may cause more or less local irregularity in the arrangement of the pits. 



Length of a left valve 0.65 mm., greatest height of same 0.33 mm., 

 length of a right valve 0.63 mm., greatest height of same 0.32 mm. 



A number of living and European Tertiary species of this type are 

 known, but we could not satisfy ourselves that the exact specific match 

 of C. tuomeyi has been described heretofore. Of American species the 

 closest allies known to us are figures on Plate XXXVI. Among these 

 C. nitiduld and C. calvertensis are probably nearer than C. porcella and 

 C. hurnsi, agreeing better in their outlines and size, and in having a 

 flattened border at their ends. They exhibit, however, appreciable 

 differences even in the points of outline and surface contour, and a 

 more obvious one in their surface ornamentation. In the last respect 

 C. tvomeyi agrees rather better with C. hurnsi but that species com- 

 monly attains a much larger size and has nearly equal instead of de- 

 cidedly unequal ends. 



Occurrence. — Choptank Formation. Peach Blossom Creek, 3 miles 

 southwest of Easton. Chesapeake Group. Yorktown, Va. 



Collection. — U. S. National Museum. 



Cythere porcella n. sp. 

 Plate XXXVI, Figs. 26-33. 

 Description. — Carapace rather full and often appearing quite smooth, 

 somewhat obliquely acuminate ovate, the anterior outline more or less 

 produced below, the posterior outline much narrower, most prominent 

 about the middle, and with the projection emphasized by usually three, 

 occasionally four or even five minute marginal spines; length about 

 0.85 mm., greatest height about 0.45 mm. Eight valve with the hinge 



