FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 147 



ventral convexity ; its hinge-area is very narrow, almost linear ; no beak per- 

 ceptible. 



Surface smooth, with a few concentric lines of growth. Prof. Hall states, 

 that in well preserved specimens, traces of very fine concentric, as well as radi- 

 ating striae have been observed. Our shells of this species are all entirely 

 silicified, in which condition the finer surface-markings are usually obliter- 

 ated. 



Formation and Locality. Found in the Corniferous limestone at the Falls of the Ohio, on the 

 Indiana shore of the river. 



Strophodonta perplana. COXRAD. 



Plate XVIII., figure 17. 



Strophomena perplana, Conrad. Jour. Acad. Nat. Sci. Phil., Vol. 8 1842. 

 Strophomena pluristriata, Conrad, ut. sup., p. 259. 

 Strophomena crenistria, Hall. Kep. 4th Geol. Dist. N. Y., p. 1711843. 

 Strophomena (Strophodonta) fragilis, Hall. 10th Rep. on St. Cab., p. 143 1867. 

 Strophodonta fragiiis, Hall. Rep. Geol. Surv. of Iowa, p. 496 1858. 



Shell semi-elliptical ; length varying from two-thirds to three-fourths the 

 width, which is from half an inch to two inches ; slightly concavo-convex, and 

 often nearly flat ; hinge-line equal, or often a little greater, than width of shell 

 below ; the cardinal extremities usually somewhat salient, except in extremely 

 old shells. Margins of the shell often a little concave just below the cardinal 

 extremities, making the width there less than below. From this incurvation 

 downwards, the lateral and basal margins form usually a regular curve. 



Ventral valve very little convex, the greatest convexity above the middle of 

 its length, from where it slopes in a gentle curve to the lateral and basal mar- 

 gins ; towards the cardinal angles it slopes a little more rapidly, and as the 

 extremities are somewhat deflected, the valve becomes slightly concave between 

 them and the umbonal region ; the apex is scarcely rising above the hinge-line, 

 and slightly incurved. 



Dorsal valve is gently concave, but often almost flat. Area of the ventral 

 valve is usually less than a line in width, and covered with vertical striae. 

 Area of the dorsal valve about half as wide as that of the ventral. 



Surface covered by fine sub-equal striae, those of the ventral valve being 

 finer than the striae on the dorsal, extremely sharp, and often gently undu- 

 lating, increasing both by bifurcation and by intercalation, and crossed by fine, 

 even, concentric striae. Prof. Hall states, that in some specimens the longitu- 

 dinal atriae rise at frequent and regular intervals into minute granules, which 

 he Considers the bases of minute spines, formerly covering the ventral valve. 

 This feature is not indicated on any of our specimens as far as I know. The 



