FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 141 



Streptorhynchus subplaims, CONRAD. 



Plate XXIX., figures 11 and 12. 



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

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

 Streptorhynchus subplana, Hall. Trans. Alb. Inst., Vol. 4, p. 226 1863. 

 Streptorhynchus subplana, Hall, llth Indiana Keport, p. 288 1881. 



Shell of medium size; plano-convex, semi-elliptical; length and width often 

 equal, but sometimes the latter exceeds the former considerably ; hinge-line 

 is generally longer than the width of shell below ; cardinal extremities are 

 always somewhat produced, but in some instances they become even nmcro- 

 nate. 



Ventral valve moderately convex ; its greatest convexity a little above the 

 middle, from where it slopes gently to the lateral and basal margins ; the car- 

 dinal extremities are deflected and forming a slight depression, extending from 

 the cardinal line obliquely to the lateral margins, intersecting these about one- 

 third the length of the shell from the cardinal extremities ; beak is small and 

 not incurved. The foramen is closed by a deltidium. The area extends to the 

 extremities, but is narrow. 



Dorsal valve is less convex than the other ; it is in our specimens almost 

 plane, with the exception of the umbonal region and the cardinal angles, both 

 of which are slightly convex, the umbonal region by a natural elevation, and 

 the cardinal angles by a downward deflection of the extremities. 



Cardinal area narrow, but about as large as the area of the other valve. 

 Both valves meet under an angle of about sixty degrees. 



The surface is covered by numerous single thread-like radii, which increase 

 by intercalation but not by bifurcation. The intercalated striae are smaller 

 than the original ; they become equal at the margin. These radii are crossed 

 by fine concentric striae, and by a few marked concentric lines of growth, 

 generally two in number, dividing each valve into three concentric zones. Our 

 specimens differ somewhat in shape from those found at Waldron, Indiana. 

 Ours are somewhat wider in proportion to length, and the extremities are more 

 produced. The size of different specimens differs greatly; the one illustrated is 

 of about maximum size, as found in our rocks. 



Formation and Locality. Occurs in the Niagara rocks in the quarries south-east of the city of 

 Louisville, Ky., where it is a rather rare species. Very fine specimens are found abundantly in the Niag- 

 ara rocks of Waldron, Indiana. 



