FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 



This species is subject to some variations in different localities ; but its nar- 

 row, almost linear area, its mucronate cardinal extremities, its shallow sinus 

 and slightly elevated fold, both of which are covered by dichotomizing plica- 

 tions, serve to distinguish it from all other Spirifera with which it is asso- 

 ciated. It might be confounded with Spir. parryanus, which has its sinus 

 and fold also covered by dichotomizing plications* but which has only single 

 ribs on its lateral parts. 



Formation and. Locality. Pound in the rotten hornstone in upper portion of the Devonian for- 

 mation, at and around the falls, in Kentucky and Indiana. It is an exceedingly rare species ; the only 

 specimen known as found in our rocks was found by Major Wm. J. Davis, on the Indiana shore of the 

 river, just below the Falls of the Ohio. 



Spirifera medialis. HALL. 



Plate XXVI., figures 2 to 5. 



Detthyris medialis, Hall. Geol. Kep. 4t,h Dist. N". Y., p. 2081843. 

 Spirifer medialis, Hall. Tenth Report on State Cab., p. 164. 



Compare Delthyris audacula, Conrad. Jour. Acad. Nat. Sci. Phil., Vol. VIII., p. 262. 

 Spirifera medialis, Hall. Pal. N. Y., Vol. IV., p. 2271867. 



Shell of medium size or larger ; sub- triangular, semi-circular or semi-ellip- 

 tical ; hinge-line longer than greatest width of shell below; cardinal extremities 

 extended, often mucronate; shell moderately convex or ventricose. Surface 

 plicated. 



Ventral valve somewhat more convex than the dorsal ; beak prominent, 

 elevated above the hinge-line, and only slightly incurved at the apex ; cardinal 

 area in our specimens of medium size, having the shape of an irregular pen- 

 tagon, of which the two sides at the cardinal extremities, in the specimen 

 before me, measure about two lines ; its margins, sloping down from the beak, 

 are almost straight, but sharply denned ; the area is almost flat except near 

 the beak, where it is slightly curved ; it is covered by longitudinal striae, and 

 divided in the middle by a triangular fissure, which is twice as high as wide at 

 its base, and which reaches to the apex of valve. Mesial sinus is of moderate 

 width, but rather deep ; it reaches to the apex, and' is generally rounded but 

 sometimes flattened at the bottom ; its margins are angular and well defined. 



Dorsal valve moderately convex or gibbous ; the greatest convexity above 

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

 gins, except at the cardinal angles, where it deflects a little, and forming a 

 slight depression ; beak small and slightly incurved ; area linear and well 

 defined. Mesial elevation prominent and sharply defined, rising abruptly at 

 its sides, rounded or slightly flattened on top. Surface ornamented by about 

 twenty-five rounded, simple plications on each side of the mesial fold or sinus, 

 of which the principal ones are sometimes marked by a small thread-like 



