Maryland Geological Survey 461 



large, convex, the extremity being nearly a right angle and on line with 

 the posterior extremity of the body ; hinge-line straight, equal to greatest 

 width of shell. Surface of body ornamented with coarse, radiating ribs 

 which are subequal on the anterior, and alternating on the central and 

 posterior parts ; the radiating ribs are crossed by finer concentric striae 

 and irregular growth lines; surface of posterior wing ornamented with 

 alternating ribs which are somewhat finer than those of the body and 

 crossed by concentric strias, the two sets of markings giving a cancellated 

 eifect. 



Eight valve and interior not seen. The description is from a single left 

 valve, which is figured. The label has been lost but the specimen is no 

 doubt from the Helderberg as is shown by other fossils in the rock mass. 



Length about 5 cm. ; height 5.3 cm. 



Occurrence. — Helderberg Formation. Locality unknown. 



Collection. — Maryland Geological Survey. 



[Ohern.] 



Aviculopecten tenuilamellatus (Hall) 



Plate LXXVIII, Fig. 4 



Avicula tenuilamellata Hall, 1859, Nat. Hist. N. Y., Pal., vol. Hi, p. 281, pi. li, 

 figs. 1 and 2, 1861. 



Description. — "Shell orbicularly subovate: left valve scarcely convex; 

 right valve flat; hinge-line straight, shorter than the greatest width of 

 the shell : anterior wing small, short, acute, separated from the body by a 

 deep narrow sinus, not extending as far forward as the anterior margin of 

 the shell; posterior wing short, broader than the anterior, acute at the 

 extremity, not extending to the line of the posterior margin of the shell. 

 Surface marked by a few unequal concentric wrinkles, and by fine, closely 

 arranged, elevated, subimbricating, lamellose stris, which extend over 

 the wings in like manner. Central portion of the shell marked by faint 

 radiating strijE." Hall, 1859. 



This shell is cited by Hall as occurring in the Lower Helderberg rocks 

 of Albany and Schoharie counties, New York. The individual at hand is 

 a badly weathered left valve probably of this species. 



