Maryland Geological Survey 297 



Order PROTREMATA 



Superfamily ORTHACEA 

 Family ORTHIDAE 



Genus ORTHOSTROPHIA Hall^ 



Orthostrophia strophomenoides (Hall) 

 Plate LIII, Figs. 20-23 



Orthis strophomenoides Hall, 1857, Tenth Ann. Rept. N. Y. State Cab. Nat. 



Hist, p. 46. 

 Orthis strophomenoides Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 177, pi. 



xiv, fig. 2. 



Descripiion. — " Shell transverse, somewhat semioval : ventral valve 

 flattened convex, with a distinct narrow mesial elevation passing from 

 beak to base ; beak scarcely distinct from hinge-line, straight : dorsal valve 

 more convex than the opposite, most elevated between the middle and the 

 beak, from which a distinct narrow depression extends towards the front; 

 beak more prominent than the opposite, obtuse, incurved: hinge-line 

 straight, nearly equalling the greatest width of the shell ; area large, plane ; 

 foramen apparently closed. Surface marked by coarse radiating striae, 

 which increase by implantation and bifurcation : several of those on the 

 mesial elevation of the ventral valve appear to coalesce along the center, 

 before reaching the beak. In well-preserved specimens, strong concentric 

 stri;T3 are visible in the depressions between the radiating striae. Shell 

 marked by a few strong concentric undulations of growth." Hall, 1857. 



Length about 1.6 cm.; width about 3.3 cm. 



This species, like Spirifer macropleurus, is a conspicuous Silurian 

 straggler. 



Occurrence. — Helderberg Formation, Xew Scotland Member. 

 Corriganville, near Cumberland, Dawson. 



Collection. — Maryland Geological Survey. 



' See Hall and Clarke, 1892, Nat. Hist. N. Y., Pal., vol. viii, pt. i, pp. 199, 223, 



