MARYLAND GEOLOGICAL SURVEY 283 



degree of obliquity of the body of the shell to the direction of the hinge- 

 line. Surface of the shell ventricose, and often subcarinate on the 

 umbones and towards the beaks, gradually and somewhat regularly sloping 

 to the basal margin, becoming attenuate and compressed toward the 

 postero-cardinal region, and abruptly truncate and even impressed on the 

 anterior side. Beaks acutely pointed, strongly incurved, terminal and 

 projecting above the line of the hinge ; posterior end at right angles to the 

 hinge straight or rounded, or sometimes sloping obliquely backwards to 

 the postero-basal margin ; base sharply rounded. Anterior border of the 

 valves excavated below the beaks, forming a rather large byssal opening, 

 which is usually about half as wide as long when the valves are united. 



" Surface of the valves marked by strong, radiating ribs, which are 

 simple throughout, strongest on the body of the shell, and becoming finer 

 on the postero-cardinal region. On the upper portion of the shell the 

 ribs are flattened on the top, and often grooved in the center, giving them 

 a strongly duplicate character, but becoming smooth below, the spaces 

 between as narrow, or much narrower, than the width of the rib. The ribs 

 are crossed by fine, concentric, imbricating lines of growth, which undu- 

 late as they cross the elevation." Hall and Whitfield, 1875. 



Occurrence. MARTINSBURG SHALE (Fairview division). Tuscarora 

 Mountain, one and one-half miles southeast of McConnellsburg, Pennsyl- 

 vania. Maysville group of the Ohio Valley and Canada. 



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



BYSSONYCHIA PRAECURSA Ulrich 

 Plate LVII, Figs. 28, 29 



Byssonychia praecursa Ulrich, 1893, Geol. Surv. Ohio, vol. vii, p. 633, pi. xlv, 

 figs. 1, 2. 



Description-. In outline and number of costae this species is quite 

 similar to Byssonychia radiata with which it is associated, but it is less 

 oblique, the hinge is longer, and the central part of the valve is somewhat 

 narrower. The marked difference between the two, however, lies in the 

 flattening of the anterior side in B. praecursa. 



