FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 149 



Strophodonta plicata. HALL. 



Strophodonta plicata, Hall. 13th Kep. on St. Cab., p. 90 1860. 



Strophodonta plicata, Hall. Pal. N. Y., Vol. 4, p, 114, pi. 63, figs. 30-321867. 



Shell of medium size ; semi-oval or semi-elliptical ; concavo convex ; hinge- 

 line somewhat shorter than the greatest width of shell ; cardinal extremities 

 rounded ; lateral margins almost straight or slightly curved ; basal margin 

 broadly curved. Shell resembling closely, as far as form is concerned, speci- 

 mens of Strophodonta demissa, of medium size and medium convexity. 



Ventral valve moderately convex, with greatest convexity at about the mid- 

 dle of the shell, from where it slopes gently to the cardinal and lateral margins, 

 and somewhat more rapidly to the front. The umbo is small ; beak little 

 elevated and incurved ; cardinal area small, with a triangular fissure, closed by 

 the beak of opposite valve. 



Dorsal valve moderately concave, nearly following the contour of ventral 

 valve ; its cardinal area very narrow, almost linear ; its beak closing the ventral 

 foramen. 



The surface is ornamented by strong, sub-angular or rounded radiating 

 striae, which increase in number by bifurcation and intercalation, and which 

 are crossed by a few concentric lines of growth near the margin. This shell 

 resembles, as before stated, Stroph. demissa, from which it is, however, easily 

 distinguished by its strong and prominent striae ; there is about the same pro- 

 portion in strength between the striae of St. plicata and Stroph. demissa, as 

 between the striae of Atrypa aspera and Atr. reticularis. 



Formation and Locality. Occurs in the Corniferous limestone of the Devonian formation at and 

 around the Falls of the Ohio, in Kentucky and Indiana, where it is, however, a very rare shell. 



Strophodonta striata. HALL. 



Strophomena striata, Hall. Geol. Kep. of 4th Dist. N. Y. 1843. 

 Leptaena striata, Hall. Pal. N. Y., Vol. 2, p. 2591852. 

 Strophodonta striata, Hall. 28th Kegent's Rep., p. 1521879. 

 Strophodonta striata, Hall, llth Geol. Report of Ind., p. 2901881. 



Shell semi-elliptical ; hinge-line equal or a little longer than the greatest 

 width of the shell below ; dorsal valve slightly convex ; ventral valve flat or 

 somewhat concave. Surface covered by unequal, radiating striae, of which 

 the stronger ones have large interspaces, which are occupied by one, mostly 

 by two, finer radii, all of which increase in number by intercalation, and not 

 by bifurcation. There are, sometimes, fine concentric lines of growth crossing 

 the radii, but these are often obsolete. The specimens of this species found 

 in our rocks differ somewhat in form from those found at Waldron, Indiana. 



