312 Systematic Paleontology 



geniculate towards the front. Area narrow. Foramen none, or a narrow 

 elevation or callosity in place of it. 



" Surface marked by very prominent sharp angular ridges and inter- 

 mediate fine undulating striae, which cover also the slopes of the ridges. 

 Sometimes a few short wrinkles mark the spaces between the ridges, along 

 the cardinal margin towards the hinge extremities." Hall, 1859. 



Length about 1.5 cm.; width about 2.4 cm. 



A very convex variety is here illustrated. Other specimens are more de- 

 pressed, bi;t both forms have the sharp angular plications covered with 

 finer strias. 



Occurrence. — Helderberg Formation, Coeymans Member. Dawson, 

 Devil's Backbone. New Scotland Member. Dawson, Devil's Backbone. 



Collections. — Maryland Geological Survey, American Museum of 

 Natural History. 



Stropheodonta varistriata (Conrad) 

 Plate LIX, Figs. 1,2 



Strophomena varistriata Conrad, 1842, Jour. Acad. Nat. Sci. Phila., vol. viii, 



p. 255, pi. xiv, fig. 6. 

 StrophodGnta varistriata Hall, 1859, Pal. N. Y., vol. iii, p. 180, pi. viii, figs. 



1-16; pi. xvi, figs. 1-8, 1861. 

 Stropheodonta varistriata Schuchert, 1897, Bull. U. S. Geol. Survey, No. 87, 



p. 427. 



Description. — " Shell semioval, varying in form from length and width 

 equal to length greater or less than the width: hinge-line equal to or 

 greater than the width of the shell below ; extremities rounded or salient. 

 Dorsal valve fiat, or more or less concave according to the convexity of 

 the ventral valve, but not conforming entirely to the curvature of the 

 latter. Ventral valve varying from slightly convex to gibbous, and some- 

 times abruptly arching towards the front; umbonal region more or less 

 prominent; beak usually a little elevated. Area narrow, almost linear. 

 Foramen linear or none. 



" Surface often finely and evenly marked with straight or slightly un- 

 dulating striae; more often with prominent sharp striae at more or less 

 equal distances from each other, and the intermediate spaces by minute 



