MARYLAND GEOLOGICAL SURVEY 253 



meeting at their inner ends ; muscular scars not visible in any specimens 

 examined. 



" Surface of both valves ornamented with rather coarse radiating 

 striae, most of which bifurcate once or oftener, while occasionally a 

 shorter one is intercalated between two longer ; crossing the whole, occa- 

 sional small marks of growth, and finer but obscure, concentric striae 

 may be seen, by the aid of a lens, on well-preserved specimens. 



"Length of a rather large specimen, 0.65 inch; breadth, 0.88 inch; 

 convexity, 0.30 inch." Meek, 1873. 



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

 one and one-half miles northeast of Fort Loudon, and Tuscarora Moun- 

 tain, two and one-half miles southeast of McConnellsburg, Pennsylvania. 



This species has hitherto been recorded only from the upper part of 

 the Fairview formation in the Ohio Valley. 



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



Genus PLECTAMBONITES Pander 



PLECTAMBONITES PISUM Euedemann 



Plate XLVIII, Figs. 1-7 



Plectambonites pisum Ruedemann, 1902, Bull. New York State Museum, 

 No. 49, p. 19, pi. i, figs. 8-20. 



Description. " Shell small, semicircular in outline, with subauricu- 

 late cardinal extensions ; highly concavo-convex, the convexity surpassing 

 that of a hemisphere; toward the cardinal ears becoming depressed con- 

 vex ; length to width as 4:5; greatest width along the hinge line, which 

 is nearly straight. Surface marked with very fine striae, which usually 

 are interrupted by from 16 to 20 coarse striae; sometimes the fine striae 

 become nearly obsolete, leaving the interspace between the coarse striae 

 almost smooth; at other times the coarse striae disappear, leaving the 

 shell uniformly and finely striated ; a few concentric growth lines are also 

 present. Pedicle valve extremely gibbous, the greatest elevation being 

 in the central part; the unbonal part sloping abruptly; the umbo being 



