ORISKANY SANDSTONE. 409 



Rensseliieria suessana. 



Plate CVII. Fio. 1 - 15. 

 Mef;anteris siuisana : Hall, Regents' Report for 1856, p. 100; Palasozoic 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 graduallj"^ 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 belowj 

 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 i.s marked by simple radiating striae, 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. 



