MARYLAND GEOLOGICAL SURVEY 255 



PLECTAMBONITES RUGOSTJS (Meek) 

 Plate LIV, Figs. 31-33 



Leptaena sericea var. rugosa Meek, 1873, Pal. Ohio. vol. i, pt. 2, pi. v, 



figs. 3f-h. 

 Plectambonites rugosa Foerste, 1912, Bull. Sci. Lab. Denison University, 



vol. xvii, p. 123, pi. i, figs. 7a-c; pi. x, figs. 7a-d. 



Description. The form of Plectambonites found so abundantly in the 

 Eden shales of the Cincinnati area was long ago separated by Meek as a 

 variety of the ubiquitous species P. sericea. " The term rugosa was given 

 not on account of the oblique wrinkles along the hinge-line, but on 

 account of the roughened surface of the general exterior surface of the 

 valves, especially anteriorly. This roughened surface appears due to the 

 presence of numerous very thin overlapping films of shell material. These 

 films appear to consist of the same extremely fine, silky, fibrous material 

 as that forming the compact body of the valves. Sometimes they are 

 traversed by the same radiating striae as those seen on that part of the 

 exterior surface of the valves where the films are not present. The films 

 may be more or less discrete from one another, but in some specimens they 

 are built up into a solid mass, resulting in a thickening of the valves 

 exteriorly. At the exterior margin of the pedicle valve, this thickening 

 may reach a total of fully 2 mm., and frequently the anterior, more or 

 less vertical slope of this thickening is crossed by lines evidently corre- 

 sponding to the extensions of the radiating striae. The thickening 

 usually is confined to the anterior half or third of the valves. It may 

 result in a succession of concentric bands, the one nearest the anterior 

 margin being the most conspicuous. At other times, the thickening 

 increases evenly, without any concentric banding, but, most frequently 

 it is more or less irregular, the films being more or less warped or broken 

 into shreds." Foerste, 1912. 



Occurrence. MARTINSBURG SHALE (Eden division). Jordans Knob, 

 one and one-half miles northeast of Fort Loudon, Pennsylvania, and in the 

 same horizon on Rickard Mountain, Maryland. Eden shale of the Ohio 

 Valley. 



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



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