MARYLAND GEOLOGICAL SURVEY 241 



usually nearly equal, simple, subangular, radiating costae, which are 

 crossed by exceedingly delicate, concentric lines of growth. Pedicle valve 

 strongly convex, subangular along the median line, with the greatest 

 elevation on the umbo. Cardinal area very high, more or less concave, 

 striated longitudinally and transversely, divided by a very narrow delthy- 

 rium, whose apical third is occupied by a flat, concave or convex deltidium. 

 Brachial valve nearly flat, slightly elevated at the beak, from which point 

 the surface slopes gradually into a broad, scarcely perceptible, rarely well- 

 defined, median sinus. Cardinal area nearly one-third as wide as that of 

 the pedicle valve, flat, divided by a triangular delthyrium, which is as 

 broad as long and more or less covered by a convex chilidium, the anterior 

 margin of which is concave. The dimensions of a rather large specimen 

 are: Length, 19 mm.; width, 20 mm.; thickness, 10.5 mm." Weller, 

 1903. 



A characteristic fossil of the Black Eiver group of the Mississippi and 

 Appalachian valleys. 



Occurrence. CHAMBERSBTJRG LIMESTONE (Mdulites bed). Wilson 

 and Pinesburg, Maryland. 



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



Genus PLECTORTHIS Hall and Clarke 

 PLECTORTHIS PLICATELLA Hall 



Plate LVII, Figs. 4-7 



OrtMs plicatella Hall, 1847, Pal. New York, vol. i, p. 122, pi. xxxii, fig. 9. 

 OrtMs plicatella Meek, 1873, Pal. Ohio, vol. i, p. 108, pi. viii, fig. 7. 

 Plectortttis plicatella Hall and Clarke, 1892, Pal. New York, vol. viii, pt. i, 

 p. 22, pi. v, figs. 18-20. 



Description. " Broadly semiov'al, nearly equivalve, length and breadth 

 about as 3 to 4; surface marked by strong radiating plicae, which are 

 usually simple, about 20 to 28 on each valve, crossed by simple elevated 

 concentric lines, which are more distinct in the depressions between the 

 costae, and often obscure or obsolete upon their exposed surfaces; valves 

 nearly equally convex, without sensible depression or elevation on either 

 one, meeting at the edges in a straight line ; cardinal line not extending 



