FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 6t 



which are sub-auriculate and flat or slightly concave; umbo moderate, and 

 beak small and pointed, little elevated above the hinge-line, but perceptibly 

 incurved over ^the latter ; hinge-area small, but increasing towards the beak, 

 and concave. Dorsal valve concave, corresponding in its general form with 

 the convexity of the other valve ; hinge area, small and narrow, almost linear ; 

 no beak perceptible. 



The surface of the ventral valve is covered by peculiar radiating striae, 

 which are strong, rounded or subangular, and simple in the umbonal region, 

 outside of which they bifurcate and increase also by implantation, but all 

 the striae on the marginal part are greatly finer than the umbonal ones ; 

 within the cardinal angles and along the cardinal margins the striae becomes 

 very irregular and often entirely obsolete. Hinge-line marked on each side 

 of the beak by four or five strong, tubular spines, which are directed obliquely 

 outwards. In the specimen before me, which completely agrees in all other 

 points with Prof. Hall's description, neither these spines nor any marks of 

 their former existence are seen ; it is possible they became obliterated by the 

 process of silicification. 



Formation and Locality. Found in the rotten hornstone in the upper strata of the Devonian for- 

 mation, at the Falls of the Ohio, on the Indiana shore of the river. Good or perfect specimens are ex- 

 tremely rare. 



Chonetes subquadrata. x. SP. 



Shell as chonetes of medium size ; sub-quadrate, hinge-line somewhat shorter 

 than the greatest width of shell ; cardinal extremities rounded ; lateral mar- 

 gins slightly curved, almost straight, except in their basal part, which is regu- 

 larly curved into the basal margin ; central half of front is straight or only 

 slightly curved. 



Ventral valve only moderately convex in its central portion, which curves 

 regularly from its middle to apex and base ; the slope towards the lateral and 

 cardinal margin is more abrupt, causing a flattening of the valve along the 

 lateral borders, and producing between the cardinal extremities, which are a 

 little deflected, and the umbo, a shallow concavity ; umbo sharply defined, 

 and moderately elevated ; the beak small, pointed, and incurved over the 

 hinge-area ; the area is small, forming a low triangle, which is divided by a 

 small triangular fissure ; the foramen is partly closed by the cardinal process 

 of the opposite valve. The margins of the cardinal area are provided with two 

 round, tubular spines on each side of the beak, which appear from their 

 stumps to have an outward direction. The dorsal valve is concave, correspond- 

 ing in its depression with the convexity of the ventral valve ; its hinge-area is 

 narrow or linear. 



The surface of both valves is covered by fine rounded, or sub-angular radi- 



