100 [Senate 



to the apex of the beak : dorsal valve flat or depressed-convex. 

 Surface smooth, or only marked by very obscure lines of growth. 



The plano-convex form and smooth surface of this species will at once 

 distinguish it from all its known congeners in the rocks of this State. It 

 is possible there may be very fine obscure radiating stri^ on perfectly 

 preserved specimens of this species, though the only individual I have yet 

 seen appears to have been smooth. 



Geological position and localilij. Shaly limestone of the Lower Helder- 

 berg group, Albany county. 



Megantekis SUESSANA. 



Pal. N.Y. Vol. iii, pi. lOG a, f. 1. 



Shell longitudinally obovate, varying to oval or subelliptical, 

 somewhat compressed ; valves nearly or quite equal ; no traces 

 of a sinus on either valve ; front narrowly rounded -, lateral 

 margins very abruptly inflected : ventral valve depressed con- 

 vex, 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 tw^o small deltidial pieces, 

 which, together with the thickened dental apophyses of the 

 opposite valvC; 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 : hinge line nearly straight, or sloping from the beak 

 at a very obtuse angle, much less than the width of the shell. 

 Surface usually appearing to be smooth, but, on well-preserved 

 specimens, remains of very faint simple radiating stria? may be 

 seen towards tlie margin, wliich always become obsolete above. 



Geological position and locality. Oriskany sandstone, Maryland. 



