ORISKANY SANDSTONE. 4 61 
Renssdseia marylaiidica (n. s.,. 
Plate CVIII. Pig. 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 
