396 Systematic Paleoxtology 



Spirifer mackopleueus (Conrad) 

 Plate LXYIII, Figs. 7, 8 



Delthyris macropleura Conrad, 1840, Fourth Ann. Rept. N. Y. Geol. Surv., 



p. 207. 

 Spirifer macropleurus Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 202, pi. 



xxvii, figs, la-lp; pi. xxvili, figs. 8a-Sd, 1861. 



Description. — " Sliell large, with about six broad rounded ribs finely 

 striated longitudinally; middle of the superior valve with a very broad 

 and prominent rib, and there is a corresponding depression on the opposite 

 valve." Conrad, 1840. 



" Shell large, varying from semielliptical to semicircular or transversely 

 elliptical, ventricose: valves nearly equally convex; hinge-line often 

 scarcely equalling the greatest width of the shell. Area narrow. Foramen 

 large. Ventral valve with a broad, deep curved sinus and three strong 

 rounded plications on each side : beak moderately elevated above the oppo- 

 site, and abruptly incurved. Dorsal valve with a broad rounded mesial 

 fold and two strong rounded plications, with sometimes a third one on 

 each side. Surface marked by fine closely arranged radiating strige, which 

 are crossed by finer concentric ones (the latter rarely visible)." Hall, 

 1859. 



This well-known Spirifer, the guide fossil for the New Scotland horizon, 

 is distinguished from all other Maryland species by its large size, few plica- 

 tions, and the finely radially striated surface. It is related to <S\ eudora 

 of the Niagara fauna. 



S. macropleurus was originally described from New York. The Mary- 

 land specimens differ very little from them and only in the plications, 

 which, as a rule, are somewhat more abruptly elevated, with an additional 

 one on each side of the fold and sinus. 



It is found in profusion at all localities at which the cherty beds of the 

 New Scotland member are exposed. 



Length about 4 cm. ; width about 6.5 cm. 



Occurrence. — Helderbeeg Formation, New Scotland Member. 

 Corriganville, Devil's Backbone, 21st Bridge, Maryland; Keyser, Cherry 

 Eun, West Virginia; Warren Point, Pennsylvania. 



Collections. — Maryland Geological Survey, U. S. National ]\Iuseum. 



