CAMBRIAN BRACHIOPODA WALCOTT . 89 



erately convex, with the apex, hi a specuiien 9 mm. long, 1.5 mm. 

 from the posterior margin. The general outHne is subcircular, with 

 the length and width approximately the same. Dorsal valve with 

 the apex marginal. The original convexity of the valves is un- 

 known, as all of the specimens are flattened in the shale. 



Surface marked by concentric ridges and striae of growth and a 

 few obscure, rounded, radiating ridges. In addition, there is a fine 

 granulation of the type of that of A. coriacea Linnarsson [1876, p. 

 21], the irregular, more or less inosculating, minute, rounded ridges 

 having fine tubercles upon them. 



A large ventral valve has a length and width of 11 mm. Other 

 specimens of the same size occur, although the average size is about 

 8 mm. Substance of the shell is unknown. 



One or two poor interiors of the ventral valve show a small 

 visceral cavity and rather slender main vascular sinuses that appear 

 to originate beside the pedicle opening. The latter opens on the 

 back slope of the apex and enlarges as it passes through the shell. 

 The interior of the dorsal valve shows a rather strong central ridge 

 that extends beyond the center of the shell ; also a rather small car- 

 dinal scar on each side of the median ridge close to the posterior 

 margin. 



Observations. — This shell is of the general type of Acrothelc 

 matthczvi (Hartt) [1868, p. 644]. It differs in its larger size, and it 

 is not probable that a species would occur in the inner Appalachian 

 trough which is present in the sediments near the margin of the 

 Atlantic basin. The fauna associated with A. yorkensis is of the 

 Middle Cambrian type of the interior trough and quite unlike that 

 of the Paradoxidcs fauna of the Atlantic Coast Province. In size 

 this species approaches A. (Redlichella) gramdata (Linnarsson) 

 [1876, p. 24], but it differs from that species in its minutely granu- 

 lated surface. 



Formation and Locai^ity. — Middle Cambrian : Argillaceous 

 shales in railroad cut alongside of gas-house, city of York, York 

 County, Pennsylvania. 



REDLICHELLA, new subgenus of ACROTHELE 



This subgenus of Acrothele has all the external characters of the 

 type species of Acrothele, A. coriacea Lyinnarsson [1876, p. 21]. It 

 differs in the interior markings of the valves. The visceral area of 

 the ventral valve is large and clearly defined, as in Botsfordia gran- 

 ulata (Redlich) [1899, p. 5], and the main vascular sinuses are very 

 strong and close to the pedicle aperture, as in the latter species. In 



