16 A COXTKIBUTIOX TO THE LOWLR DEVONIAN FAUNAS OF MARYLAND 



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. 



MyTILARCA CORDIFORMIS (Plall) ? 



Plate LXXV, Figs. 5, 6 



Megam'bonia cordiformis Hall, 1859, Nat. Hist. N. Y., Pal., vol. iil, p. 278, 

 pi. 1, figs. 6a. b, 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- 

 Jate, the prominence covering the muscular impression small. Surface 

 marked by concentric lamellose strife." Hall, 1859. 



Two specimens in the collection of Mr. George M. Eoeder 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. 



Subgenus PLETHOMYTILUS Hall 



Mytilarca (Plethomytilus) rowei n. sp. 



Plate LXXY, Fig. 7 



Description. — Shell equivalve; not strongly inequilateral; gibbous; 

 broadly ovate ; umbonal slope expanding somewhat rapidly from umbones 

 to about one-fourth or one-third the length of the shell, then arching 

 broadly and gently to the postero-basal margin, sloping gradually to the 

 postero-dorsal and suddenly to the antero-basal margin; postero-dorsal 

 margin almost rectilinear to about three-fourths the length of the shell, 

 posterior extremity subcircularly rounded, basal margin broadly rounded, 

 anterior margin rectilinear ( ?) to br^ak : 'hcaks prominent, a])proximate. 

 Surface of interior smooth, interrupt('(l irregularly by broad, often dis- 

 continuous, concentric furrows. Hinge-line not seen. 



Greatest length 73 mm.; greatest width .■).") mm. 



