A Contribution to the Lower Devonian Faunas of Maryland 15 



Genus LEPTODESMA Hall 



Leptodesma ? sp. 

 Plate LXXV, Fig. 2 



Description. — Three iiicoraplete internal casts whose outlines and other 

 characters cannot l^e made out with certainty are tentatively referred to 

 this genus. The ligamental area is long and wide and longitudinally 

 striated. Beneath the beaks are several fine transverse teeth, and posterior 

 to the beak is a hinge margin evidently corresponding to a tooth in the 

 shell. The anterior extremity seems to be neither nasute nor aurieulate. 

 The anterior muscular impression is small and deep. In the cast the 

 position of pallial line is marked anteriorly by a sharp ridge. A part of 

 an exterior from the same rock mass shows the surface of the shell to be 

 marked only by irregular concentric growth lines. 



Length 5.5 cm. ; height 4.5 cm. 



Occurrence. — Oriskany Formation, Eidgely Member. Rock Enon 

 Springs, Virginia. 



Collection. — U. S. National Museum. 



Family AMBONYCHIIDAE 



Genus MYTILARCA Hall 

 Mytilarca marylandica n. sp. 



Plate LXXV, Figs. 3, 4 



Description. — Shell inequivalve, slightly inequilateral, convex, de- 

 pressed, subtrigonal in outline; anterior side concave beneath beaks, 

 thence broadly rounded, antero-ventral margin truncate, posterior ex- 

 tremity sharply rounded to subangular. Beaks prominent, approximate. 

 Left valve convex, the maximum inflation being in the umbonal region; 

 right valve less convex, much depressed toward ventral side. Muscular 

 markings and exterior ornamentation not seen. 



Length 20 mm.; height 24 mm. 



The specimen figured is a cast of the interior, the only individual seen. 

 The truncation of the postero-ventral side seems due, in part at least, to 

 poor preservation, and the irregularity in the outline of the anterior 



