Maryland Gkological Survey 571 



Description. — "Shell transverse, often much wider than loiiji', sub- 

 plano-convex; hinge-line nearly or (juite equal to three-lnurths tlie great- 

 est width of the shell, and aliout ecjual to its length; cardinal exti'emities 

 rounded. The dorsal valve is very convex or gibbous, M'ith a deep rounded 

 sinus in the median line, abruptly curving towards the cardinal line and 

 somewhat more gently to the front, depressed and scarcely flattened at 

 the cardinal extremities. Ventral valve nearly flat, or rising from the 

 base to tlie umbo with litth- convexity; median line carinate. witli a 

 strong and defined angular ridge, front abruptly sinuate; area cnmpi'.ra- 

 tively narrow, a little curved near the beak; foramen wide. 



"Surface marked by fine radiating striae, which increase l)y liifurca- 

 tion and interstitial additions, and are strongly curved upwards to the 

 hinge-line; testure punctate. The specimens are mainly casts, and the 

 lines of growth are obscure. 



" The cast of the dorsal valve shows a wide subquadrate muscular im- 

 pression, divided by a rounded median ridge, and transversely by a low 

 ridge on either side, after the manner of 0. degantula and others of that 

 type. The socket plates are strong and very divergent, and the cardinal 

 process is apparently triplicate; the area is narrow and flat. 



" The cast of the ventral valve is nearly flat on the sides, with a defined 

 angular carina along the median line. The muscular impression is sub- 

 quadrate, deeply bilobed below, and slightly lobed at the sides. The 

 dental lamellos are strong and triangular. 



" In specimens which have suffered compression, the length is aljout 

 fhree-fouiths the width; but in some individuals the widtli is nearly 

 double the length." Hall, 1867. 



This species is characterized by a deep sulcus on dorsal valve, very 

 elevated angular fold on ventral valve, and greatest width posterior to 

 middle of valve. A single valve has been observed. 



Length 18 mm.; width 28 mm. 



Occurrence. — Jexxixgs FoR:\[ATioy, C'iie:\[uxg ]\Iembfr. Xear El- 

 lerslie, Pennsylvania. 



CoUection. — Maryland Geological Survey. 



