DESCRIPTION OF SPECIES 321 



size, the more prominent beak, and in the greater, length in proportion to 

 the width. 



Horizon: Zone of Stricklandinia 18 to 25 feet above the base of the 

 Kankakee limestone, at Elgin, and near Channahon, Illinois. 



GENU 8 VIRGIANA TWENHOFEL 



Virgiana barrandei var. mayvillerisis n. var. 

 Plate 17, Figures 3 to 7 



Description : Shell presenting a very considerable range of variation in 

 size and appearance; unequally biconvex; subovate in outline; length 

 about \y% times the width, the greatest width anterior to the middle ; 

 anterior margin rounded. All the specimens are in the form of casts of 

 the interior. 



Ventral valve strongly arcuate from beak to front, the curvature in- 

 creasingly convex toward the beak, the greatest convexity posterior to the 

 middle, the median portion elevated into a more or less distinct fold. In 

 the casts a sinus appears at the beak which soon becomes occupied by a 

 plication that divides two or three times so as to form a mesial fold in 

 the middle and anterior portions, which in some specimens is very prom- 

 inent. The surface is strongly convex transversely in the anterior por- 

 tion, becoming progressively more arcuate in the middle and posterior 

 portions to near the cardino-lateral margins, above which it is abruptly 

 curved toward the delthyrium ; cardinal area arcuate ; delthyrium large 

 and triangular; beak high, arcuate, strongly incurved over the hinge line ; 

 spondylium deep, supported by a short median septum. 



Dorsal valve gently convex from beak to front along the median line, 

 highest in the umbonal region, where it is strongly convex transversely, 

 the curvature becoming more gentle toward the anterior portion, where 

 the valve is nearly flat ; the median portion of old shells usually depressed 

 so as to form an indistinct mesial sinus, which extends from near the 

 beak to the front margin. No trace of a mesial sinus is present in the 

 dorsal valves of young specimens, but instead they frequently bear an 

 indistinct mesial fold ; beak rather prominent and incurved beneath that 

 of the ventral valve; crural impressions long, slender, and nearly parallel. 



Surface of both valves marked by rather strong radiating striae, which 

 divide two or three times between the beaks and the margins, and by con- 

 centric lines of growth, which are more numerous near the margins. 



There is great variation in the size of the shells, but the type specimen, 

 which is near the average, measured as follows : length, 214 inches ; great- 

 est width, iy 8 inches; thickness, 1% inches. 



