A Contribution to the Lower Devonian Faunas of Maryland 21 



separated from body, convex ; anterior wing not seen ; anterior extremity 

 suddenly rounded, basal margin broadly and regularly arcuate, posterior 

 extremity broadly rounded; surface of body covered by low, arched, 

 radiating ribs, between which, toward the margin, finer ones are inter- 

 posed ; posterior wing unornamented. Hinge-line straight. 



The description is from a cast of the interior of a left valve which is 

 not quite complete. The species bears some resemblance to A. recticosta 

 Hall, but the general outline and the character of the radiating ribs at 

 once separate it. Another imperfect cast in the Maryland Geological 

 Survey collection is doubtfully referred to this species. It is, however, 

 more oblique, the large ribs are smaller and the small ones larger in pro- 

 portion than the specimen described. It is not improbable that further 

 collections will separate this as another species, the material now in hand 

 not warranting such procedure. 



Length 6.5 cm.; height 4.5 cm. 



Occurrence. — Oriskany Formation, Eidgely' Member. Near Foun- 

 tain, four miles southwest of Keyser, West Virginia ; Hancock, ]\Iaryland. 



Collections. — Maryland Geological Survey, TJ. S. jSTational Museum. 



Genus AVICULA Klein 



AvicuLA recticosta Hall ? 

 Plate LXXVI, Fig. 7 



Avicula recticosta Hall, 1859, Nat. Hist. N. Y., Pal., vol. ili, p. 46G, pi. cix, 

 fig. 3, 1861. 



Description. — '' Shell slightly oblique, subrhomboidal : hinge-line 

 greater than the greatest width of the shell below; width equal to about 

 once and a third the height, very moderately convex. Posterior wing large, 

 extending nearly as far backwards as the posterior margin of the shell. 

 Anterior wing smaller, triangular, slightly concave on the outside. Sur- 

 face marked by strong dichotomizing subequal ribs, which proceed prin- 

 cipally in pairs from the umbo to the margin of the shell. Posterior wing 

 with fine radiating ribs and close concentric laminas; the anterior wing 

 being marked only by the concentric striae." Hall, 1859. 



