i8o 



Cincinnati Society of Natural History. 



Ctknobolbina subcrassa, n. sp. 



Plate VIII, Figs. 1-3. 



Size: Length, 1.15 mm.; hight, 0.7 mm.; thickness, 

 0.6 mm. 



Carapace widest in the posterior half, obliquely subovate, 

 the hinge line long and straight. Flange thick and well 

 developed along the posterior and ventral sides, weak or quite 

 obsolete in front, partly overhanging the ventral contact edge 

 of the valve and hiding a number of rather faintly marked 

 transverse depressions between them. Anterior sulcus obso- 

 lete or distinguishable only in the dorsal region, the posterior 

 one strongly impressed, especially upon the posterior side, 

 extending obliquely backward and downward more than two- 

 thirds across the valve. A small tubercle occurs in the 

 antero-dorsal corner of the posterior lobe, while a thin and 

 prominent ridge runs along the lower side of the combined 

 median and anterior lobes. Between this ridge and the flange 

 the surface is sharply excavated. Surface without ornament. 



This species finds its nearest relations in C. crassa and C. 

 fulcrata, occurring in the shales of the Black River group in 

 Minnesota. A comparison with the published figures of 

 these species will not only show this relationship, but at the 

 same time reveal several obvious differences by which the 

 species may be recognized. 



Formation and Locality. — In a thin band of shale belonging 

 to the Ridley limestone division of the Stones River group, 

 near the bottom of the Kentucky gorge, at High Bridge, Ky. 



Ctenobolbina obuqua, n. sp. 

 Plate VIII, Fig. 4. 



Size: Length, 1.1 mm.; hight, 0.7 mm. without flange, 

 0.75 mm. with flange. 



This is a moderately convex and very simple species of the 

 genus, there being a single sulcus, curved, but on the whole 

 nearly vertical, and sharply defined on the posterior side 

 only. The valves are shorter and more oblique than usual, 

 and the flange a delicate projecting plate or fill ; the surface 

 is minutely reticulate or punctate. A small tubercle is 

 situated near the middle of the antero-ventral fourth. 



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