MARYLAND GEOLOGICAL SURVEY 249 



Genus PIANODEMA Foerste 



PlANODEMA SUBAEQUATA (Conrad) 



Plate XLV, Figs. 1-3 



Orthis subaequata Conrad, 1843, Proc. Acad. Nat. Sci. Philadelphia, vol. i, 

 p. 333. 



Dalmanella subaequata Hall and Clarke, 1892, Pal. New York, vol. viii, pt. 1, 

 pp. 194, 207, 224, pis. 5c, figs. 6-11. 



Orthis (Dalmanella) subaequata Winchell and Schuchert, 1893, Geol. Min- 

 nesota, vol. iii, p. 446, pi. xxxiii, figs. 30-36. 



Dalmanella subequata Weller, 1903, Geol. Surv. New Jersey, Pal., vol. iii, 

 p. 156, pi. x, figs. 3, 4. 



Description. " Shell subequally biconvex, usually wider than long ; 

 the hinge-line shorter than the greatest width of the shell, except some- 

 times in young individuals; cardinal extremities angular or rounded. 

 Surface of each valve marked by numerous, fine, tubulose striae, which 

 bifurcate about twice in passing from the beak to the anterior margin. 

 Pedicle valve strongly and evenly convex, the greatest elevation posterior 

 to the middle of the shell; near the beak and upon the umbo no medial 

 depression exists, but near the middle of the valve a broad, shallow and 

 indistinct sinus begins and becomes deeper toward the anterior margin. 

 The cardinal area is well defined, broadly triangular, elevated and only 

 moderately concave; the delthyrium, with slightly curved sides, is about 

 t\\dce as high as wide. Brachial valve more evenly, but a little less convex 

 than the pedicle, the greatest elevation near the middle. Near the beak 

 the mesial portion of the shell is usually flattened or slightly depressed, 

 but near the middle of the shell this flattening gradually changes into a 

 low, broad, ill-defined elevation, corresponding with the sinus of the 

 pedicle valve. The cardinal area is narrow and concave, with a delthy- 

 rium as broad or broader than high. The dimensions of an average speci- 

 men are : Breadth, 16 mm., and length, 14 mm." Weller, 1903. 



Occurrence. CHAMBERSBURG LIMESTONE (Echinospherites bed). 

 Pinesburg Station, Maryland, and various localities in southern Penn- 

 sylvania. Stones Eiver group of Tennessee and the Black Eiver group 

 of Minnesota, etc. 



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



