MARYLAND GEOLOGICAL SURVEY 243 



distance. Surface of both valves marked by unequal, angular, radiating 

 costae, about 10 or 12 of the largest ones having their origin at the beak, 

 the remainder being lateral branches from these. In some of the larger 

 individuals as many as 60 or more costae are present along the margin 

 of the shell. The branches from the main costae are small at first, but 

 increase in size towards the margin of the shell, and themselves give off 

 additional branches. In many individuals this manner of branching gives 

 to the costae a more or less fasciculate appearance, each fascicle having 

 one large rib in the center, with smaller ones on either side. In those 

 specimens having the shell well preserved the bottoms of the grooves 

 between the costae exhibit a series of fine, transverse crenulations. 

 Pedicle valve convex, subcarinate along the median line, the lateral slopes 

 nearly straight, the greatest convexity of the valve about one-third the 

 distance from the beak. Cardinal area moderately concave, forming an 

 angle of about 45 with the plane of the valve, about five or six times as 

 high. Delthyrium a little higher than wide. Brachial valve nearly flat, 

 with a sinus beginning close to the beak and expanding in a broad, shallow 

 depression towards the front. 



" The dimensions of an average-sized specimen are : Length, 8 mm. ; 

 width, 8.25 mm. ; convexity of pedicle valve, 2.5 mm." Weller, 1903. 



The particular variety of Dalmanella testudinaria described above by 

 Weller from the Trenton limestone of New Jersey is represented in the 

 Sinuites bed at the base of the Martinsburg shale. 



Occurrence. MARTINSBURG SHALE (Sinuites bed). The Trenton 

 variety here figured occurs at Chambersburg, Pennsylvania, and Stras- 

 burg, Virginia. Shells identified as this species occurs in most of the 

 Middle and Upper Ordovician strata of Europe and America. 



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



DALMANELLA EDSONI n. sp. 



Plate XLIX, Figs. 17-21 



Description. This well-marked new species is distinguished by its 

 large size and by its sharply folded plications, arranged in bundles. The 

 length and breadth of average specimens from southern Pennsylvania is 



