453 Systematic Paleontology 



Family AMBONYCHIIDAE 



Genus MYTILARCA Hall 



Mytilarca maeylandica 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. 

 Loft valve convex, the maximum inflation being in the umbonai 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 

 margin is caused by mechanical displacement of a portion of the cast. 

 The precise horizon from which this species comes is unknown. 

 Occurrence. — Helderberg Formation. Cumberland. 

 Collection. — George M. Eoeder. 



[Ohern.] 

 Mytilarca cordiformis (Hall) ? 

 Plate LXXV, Figs. 5, 6 



Megamhonia cordiformis Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 278, 

 pi. 1, figs. 6a, 6, 1861. 



Description. — " Shell cordiform, extremely gibbous from the base up- 

 wards to the umbones, which are elevated and incurved, not compressed at 

 the basal or lateral margins ; anterior cardinal extremity slightly auricu- 

 late, the prominence covering the muscular impression small. Surface 

 marked by concentric lamellose strife." Hall, 1859. 



Two specimens in the collection of Mr. George M. Roeder from an un- 

 known horizon appear to belong to this species. In both specimens both 

 valves are present. The individuals are smaller than Hall's figure but in 

 other respects are very similar. 



Occurrence. — Helderberg Formation. Cumberland. 



Collection. — George M. Eoeder. [Ohern.] 



