ORISKANY SANDSTONE. 
459 
llensselseria suossana. 
Plate CYII. Fig. 1 - 15. 
Meganteris suessana : Hall, Regents’ Report for 1856, p. 100; Palaeozoic Fossils, 1857, p. 60. 
Shell longitudinally ovate varying to oval or subelliptical, and sometimes 
the length and breadth equal, somewhat compressed : valves nearly 
or quite equal; no traces of a sinus on either valve; front narrowly 
rounded ; lateral margins very abruptly inflected. Hinge-line nearly 
straight or sloping from‘the beak at a very obtuse angle, much less than 
the width of the shell. Ventral valve depressed-convex, most prominent 
along the middle, sloping very gradually towards the sides : beak 
pointed, small, very angular along its lateral borders, incurved, rising 
above the hinge-line but not touching the other valve, perforate in the 
apex by a small round aperture partly completed by the two small 
deltidial pieces, which, together with the thickened dental apophyses 
of the opposite valve, often partially close the triangular foramen below. 
Dorsal valve symmetrically depressed-convex, sloping very gradually 
from near the middle laterally and towards the front, rounding a little 
more abruptly towards the beak, which is pointed and scarcely incurved. 
Surface smooth in silicified specimens; but on well-preserved examples 
the entire surface is marked by simple radiating strim, which are al¬ 
most always preserved in some degree towards the lateral and basal 
margins of the shell. 
Some variety of form occurs among the fossils of this species, in which we find 
the broad ovate and the symmetrically oval forms as seen in the first two figures on 
the plate. Another form has a more obtuse beak, and is contracted towards the 
front, swelling abruptly at the sides, and more gibbous in the middle. 
The interior structure has been very well shown in numerous specimens. The 
ventral valve preserves two strong teeth, with much narrower dental plates than 
in R. marylandica , and which in like manner are shown in the rostral cavity. The 
muscular area is not strongly defined, and the median septum is scarcely developed. 
In entire specimens the foramen is neatly rounded; while, in the absence of the 
deltidial plates, it is a triangular space communicating with the cavity of the shell. 
