118 SYSTEMATIC PALEONTOLOGY 



may result in a more natural and serviceable classification of the fossil 

 species than the one now in use. 



There is no described American species with which C. exanthemata 

 might be compared. Of numerous European allies, among which C. 

 aculeata and C. formosa of Bosquet are perhaps the closest, none is so 

 coarsely tuberculated. 



Dimensions of an average left valve : greatest length 0.9 mm., greatest 

 height of anterior half 0.5 mm., height just beneath postcardinal angle 

 0.38 mm. 



Occurrence. — Choptank Formation. Pawpaw Point. Calvert For- 

 mation. Plum Point. Also in the Chesapeake Group, on the James 

 Eiver, and at Yorktown. 



Collections. — TJ. S. National Museum, Maryland Geological Survey. 



Cythere rugipunctata n. sp. 

 Plate XXXVIII, Figs. 16-i:. 



Description. — This species is too much like the preceding C. exanthe- 

 mata to require detailed description. Compared with that species it is 

 distinguished at once by its peculiar surface marking, which is a com- 

 bination of irregular, twisted or wrinkled plications, commonly arranged 

 vertically across the posterior half of the valves — and unequal though 

 usually rather large pits between the plications. Another striking dif- 

 ference is brought out in comparing the outlines of the valves of the two 

 species. In C. exanthemata all of the margins except the central part 

 of the ventral portion, or it may be" its greater part, is lined with a row 

 of spines producing a dentate outline. In C. rugipunctata, on the con- 

 trary, marginal spines occur only at the ends of the valves, the posterior 

 end having three or four and the anterior margin about the same number. 



Length of a left valve 0.71 mm., height at anterodorsal angle 0.38 

 mm., height at posterodorsal angle 0.31 mm., thickness of single valve 

 0.20 mm. 



Occurrence. — Chesapeake Group. James Eiver, Va. Probably also 

 in Maryland. 



Collection. — U. S. National Museum. 



