Maryland Geological Survey 313 



equal stria;; and again in other specimens by alternating larger and 

 smaller strise, of which there are frequently three regular gradations in 

 size. Radiating striae crossed by fine concentric elevated lines, and often 

 by undulations or indentations which are more conspicuous on those shells 

 where the strias are in fascicles of finer between stronger ones. Vascular 

 impressions of the ventral valve circumscribed by lamellae, more or less 

 distinctly fiabellate: impressions of adductor muscles elongate-oval." 

 Hall, 1859. 



Length 1.1 cm. to 1.9 cm.; widtli 1.5 cm. to 2.5 cm. 



Occurrence. — PIelderberg Formation, Keyser Member. Cash 

 Valley, Maryland ; Hyndman, Pennsylvania. 



Collection. — Maryland Geological Survey. 



[Maynard.] 



Stropheodonta goeymanensis n. sp. 

 Plate LVII, Fig. 6 



Description. — Shell medium size, subsemioval; concavo-convex, hinge- 

 line of specimen observed shorter than width, transverse ventral valve very 

 convex, greatest convexity anterior to middle. Area not seen. Dorsal 

 valve not observed. Surface of ventral valve ornamented by plications of 

 unequal strength distant about 1 mm. near margin. Interior of ventral 

 valve showing large distinct flabellifonn muscle scars. 



Length 2.7 cm.; width 3.3 cm. 



This species resembles 8. incequircudiata Hall of the Onondaga, but 

 differs in its much larger muscular scars. 



Occurrence. — Helderberg Formation, Coeymans Member. Devil's 

 Backbone. 



Collection. — Maryland Geological Survey. 



[Swartz.] 



Stropheodonta demissa (Conrad) 

 Plate LVII, Fig. 11 



Strophomcna demissa Conrad, 1842, Jour. Acad. Nat. Sci. Phila., vol. viii, 



p. 258, pL xiv, fig. 14. 

 Strophodonta demissa Hall, 1867, Nat. Hist. N. Y., Pal., vol. iv, pp. 81, 101, pi. 



xl, figs. 14-17; pi. xii, figs. 1-5. 



