MARYLAND GEOLOGICAL SURVEY 125 



Length of a typical left valve 0.58 mm., height of same 0.50 mm., 

 length of a typical right valve 0.93 mm., height of same 0.50 mm., thick- 

 ness of perfect carapace 0.40 mm., length of left valve of a narrow variety 

 0.90 mm., height of same 0.47 mm., length of right valve of the short 

 variety 0.78 mm., height of same 0.47 mm. 



Occurrence. — Calvert Formation. Plum Point. 



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



Cytheridea ( ?) chesapeakensis n. sp. 

 Plate XXXVII, Fig. 9. 



Description. — Eight valve ovate-subtriangular, obtusely acuminate pos- 

 teriorly, the dorsum arcuate and curving gradually into the anterior out- 

 line, which is rather narrowly rounded in the lower half ; ventral margin 

 straight in the anterior half and gently arcuate posteriorly. Surface 

 rather strongly convex in the posterior half, sloping more gently forward. 

 Ornament, consisting of small pits arranged in six or seven curved longi- 

 tudinal rows, almost confined to the postventral third. Posterior ex- 

 tremity with a fringe of five or six small spines. Anterior edge with a 

 narrow but sharply defined flat border. Test very thin. Hingement not 

 satisfactorily determined, but it is certainly not like that of a true 

 Cythere. The cardinal edge looks like that of a Cytheridea, but we 

 failed to make out the denticles if these are really present. Possibly the 

 species belongs to Cytherideis. Left valve unknown. 



We are not entirely satisfied that the valve above described may not 

 turn out to be the right valve of a species of Cythere like our C. porcella, 

 but as the hinge, which appears to be in good condition, presented not a 

 sign of the terminal teeth and sockets observed in that and all other 

 species of Cythere, we felt obliged to classify the specimen in accordance 

 with its known characteristics. 



Length of a right valve 0.8 mm., greatest height of same 0.43 mm. 



Occurrence. — Calvert Formation. Plum Point. 



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



