Maryland Geological Survey 321 



Stropheodonta sp. 



Description. — A large Stropheodonta measuring 4 cm. in length by 

 about 4.5 cm. in width was found in the Keyser member of the section at 

 Pinto. The individual observed is too poorly preserved for specific 

 identification, and it has not been figured. It resembles 5". hecl:ii or an 

 unusually large individual of S. bi2}a'7-tita. 



Occurrence. — Helderberg Formation, Keyser Member. Pinto. 

 Collection. — Maryland Geological Survey. 



[Swartz.] 

 Genus STROPHONELLA Hall' 

 Stropiionella gexiculata (Hall) 

 Plate LIX, Figs. 3-5 



Strophodonta geniculata Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 483, pi. 

 xxill, figs. 6a, 66, 6c, 1861. 



Description. — " Shell somewhat semicircular, abruptly geniculate 

 towards the base. Dorsal valve flat for two-thirds the length of the shell, 

 when it is abruptly inflected. Ventral valve with a narrow area, the beak 

 projecting a little beyond the opposite, slightly convex from the beak 

 towards the middle, becoming flattened and concave, and the last third 

 abruptly deflected, corresponding with the opposite valve. Surface at and 

 near the beaks marked by strong radiating striae, which bifurcate, and are 

 increased by interstitial additions towards the base of the shell. Tubular 

 openings are noticed at intervals on the summits of the stronger striae, 

 and round or o\al pores mark the interstitial spaces. 



" In some specimens the center of the dorsal valve is marked by a strong 

 sinus with a prominent elevation on each side, with a corresponding mesial 

 elevation on the center of the opposite or concave valve, and a sinuosity on 

 either side." Hall, 1859. 



^ This genus is readily distinguished from Stropheodonta in having the 

 relative convexity of the valves reversed. In other words, Stropheodonta is 

 plano-convex or concavo-convex, while in Strophonella a section along the 

 middle from the beak to the anterior margin will be sigmoid in outline. 

 Otherwise the two genera are essentially alike. See Hall and Clarke, 1892, 

 Nat. Hist. N. Y., Pal., vol. vlii, pt. 1, pp. 290-293. 



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