FOSSILS Otf THE SILURIAN AND DEVONIAN BOOKS. 5i 



in some cases not much above the middle, and is little produced in front. 1 

 have a specimen before me, where the plication of the mesial sinus is even 

 overreaching the general surface of the valve ; sides curving abruptly to the 

 margins ; beak incurved, obtuse, arching over the broad fissure ; cardinal area 

 sloping on both sides down to about the middle of the valve, bounded by a 

 sharp, somewhat curved line of demarcation, having a considerable size and 

 being somewhat concave. 



Dorsal valve moderately convex and regularly curving to the lateral and 

 basal margins ; mesial fold not much elevated, and limited to the lower half 

 of the valve, where it is well defined. Surface plicated, the ribs rounded or 

 sub-angular, becoming obsolete towards the beak, and prominent in the basal 

 half. There are generally two, sometimes three plications in the sinus, and 

 three, rarely four on the fold, while the lateral slopes of the valves contain 

 from four to five. The plications are crossed by fine concentric lines of growth, 

 which sometimes at irregular intervals are crowded into squamose, imbricating 

 lines. The entire surface is finely papillose or punctate, and, when well pre- 

 served, might be mistaken for a punctate shell. The substance of the shell 

 is lamellose -prismatic and brittle. The interior of the ventral valve shows a 

 broad, short and deep spoon-shaped pit, extremity of which is bent abruptly to 

 dorsal side. The septum, supporting the conjoined lamellae, extends from 

 one-third to one-half the length of the valve, and in some examples may 

 extend still further" towards the anterior margin. This species bears much 

 resemblance to P. arata, but its plications are less numerous on the whole 

 valve ; less numerous and stronger in the mesial depression and elevation, 

 and its shell is less arcuate. 



Formation and. Locality. Found in the Corniferous limestone in the neighborhood of the Falls of 

 the Ohio in Kentucky and Indiana. Pretty abundant, but fair specimens rather rare. 



Fentamerella thusnelda. N.SP. 



Plate XXXI., figures 26, 27 and 28. 



Shell of medium size, ovoid or sub-quadrate; cardinal extremities rounded, 

 forming in the beak of the dorsal valve an angle of a little more than sixty 

 degrees ; length exceeding the width considerably, giving to the shell an elon- 

 gate, somewhat slender appearance. Ventral valve ventricose, even gibbous ; 

 convexity regular from beak to front, and also transversely ; greatest convexity 

 a little above the middle of the valve ; mesial sinus indicated by two very 

 strong plications, and by a wide and deep groove on each side of them ; the 

 summit of these plications drops not at all, or at least very slightly, at the very 

 front of the valve, below the regular surface ; the two prominent grooves ex- 

 tend almost to the beak, forming on the umbo only one rib, which separates 



