ORISKANY SANDSTONE. 461 



Rensscliieria iiiarylaiidica ( n. s.^- 



Plate CVIII. Fio. 3 a - m. 



Shell elongato-ovoid, the outline elliptical-ovate, broader above the 

 middle and gently narrowing towards the front, which is sometimes 

 compressed ; without mesial sinus or elevation. Ventral valve ventri- 

 cose, the greatest elevation at the first third from the beak, where it is 

 sometimes subobtusely angular : beak small, incurved over the opposite 

 valve, and sometimes so much incurved as to close the foramen par- 

 tially or entirely ; often, however, showing the deltidial pieces : the 

 lateral margins, in old shells, abruptly inflected. Dorsal valve less 

 convex than the opposite, the greatest gibbosity being at the first third 

 below the beak, sometimes flattened towards the front and abruptly 

 inflected at the lateral margins ; leaving, with the inflection of the 

 opposite valve, a flattened or concave space on each side. 



Surface marked by fine radiating striae, which, in the silicified specimens, 

 are often scarcely distinguishable on the upper part of the shell. 



The interior of the ventral valve shows a deep symmetrical cavity, the 

 muscular impressions occupying a narrow oval space above the middle of 

 the shell. The cardinal teeth are strong, and supported below by strong 

 dental plates, which, on their anterior edges, are separated from the side 

 of the shell, and, about halfway in the depth of the cavity, turn back- 

 wards towards the beak, come together in the rostral cavity, and reach 

 into the foramen. From the anterior basal margins of these plates proceed 

 the lamellae which border the muscular impression, and which gradually 

 become obsolete in the anterior direction : the muscular impression is 

 oblong-subelliptical, and the narrow imprints of the adductor muscles are 

 small and often but faintly defined. There is a medio-longitudinal septum, 

 which is much less strongly marked than the marginal rim. The deltidial 

 pieces are rarely preserved in the separated valves. 



The interior of the dorsal valve shows the strong hinge-plates, each 

 with a wide triangular upper surface, and connected by a transverse 



