130 SYSTEMATIC PALEONTOLOGY 



particularly striking when right valves are compared. In consequence of 

 these differences the whole outline assumes a different shape so that it 

 would be described as oblong subquadrate instead of subovate. The sur- 

 face swelling and ornament are about the same in the two varieties. 



Length of right valve 0.84 mm., greatest height of same 0.42 mm., 

 length of left valve 0.90 mm., greatest height of same 0.48 mm. 



Occurrence. — Calvert Formation. Plum Point. 



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



Cytheke spiniplicata n. sp. 

 Plate XXXVIII, Fig. 18. 



Description. — Valves moderately convex, subovate, the anterior end 

 somewhat oblique and a fifth or only a sixth wider than the posterior 

 end; entire outline, excepting the central part of the ventral edge, den- 

 ticulate, the. anterior edge being provided with a double row of flattened 

 spines; surface roughly eehinulate, the spines irregularly disposed or 

 covering the sides and crests of very irregular but on the whole vertically 

 arranged plications. In certain lights the latter appear twice inter- 

 rupted, this appearance being due to a low longitudinal central ridge and 

 two narrow sulci defining it that in other lights are obscured by the plica- 

 tions. 



Length of a right valve 0.90 mm., greatest height of same (across 

 anterior end) 0.49 mm., thickness of same about 0.25 mm. 



The rough surface and general aspect of this species is such that it 

 could not for a moment be confounded with any of the known American 

 forms, excepting possibly our C. evax ohlongula. There are, however, 

 several species in Tertiary and later deposits of Europe with which it 

 might be compared, notably C. scahropapulosa Jones, from the Eocene of 

 England, and C. scabra Miinster, from the Miocene of Germany and 

 France. Still, our species seems sufficiently distinct to render detailed 

 comparisons unnecessary. In C. evax the surface ornament is not the 

 same, though somewhat similar, and the central ridge is more prominent 

 and broken up into two parts. Beneath it there is another ridge that has 

 no counterpart in C. spiniplicata. 



Occurrence. — Calvert Formation. Plum Point (Rare). 



Collection. — U. S. National Museum. 



