MERISTELLA OF THE UPPER HELDERBERG GROUP. 
309 
Meiistella unisiilcata. 
SUBGENUS ? PENTAGONIA (Cozzens). 
PLATE L. 
Atrypa unisulcata : Conrad, Annual Report on the Palaeontology of New-York, p. 56. 1841. 
Rhynchonella unisulcata : (Conrad, sp.) Hall in Tenth Report on the State Cabinet, p. 125. 1857. 
Jltrypa unisulcata t referred with doubt to Meristella, and name Gonioc<elia proposed, in 
Fourteenth Report on the State Cabinet, p. 101. 1861. 
Compare Pentagonia peersii, Cozzens, Annals of the New-York Lyceum, Yol. iii, pa. 158, pi. x, f. 2 a, b. 
1846. 
Shell subtrigonal, quadrilateral or sometimes subhexagonal in outline, 
wider in front, with the sides sometimes sloping from the beak ; and 
in others, the hinge-line extended nearly straight, and the sides nearly 
rectangular to it. A wide mesial depression on one side, with promi¬ 
nent elevation on the other. 
Ventral valve with a broad deep mesial sinus which occupies nearly 
the whole width of the valve, and is bounded on either side by an 
angular elevation which extends from the beak to the baso-lateral 
angles. The portion of the valve outside of the limitation by the sinus 
is abruptly inflected upwards, often nearly at right angles ; the umbo 
is prominent, and the beak is incurved over the umbo of the dorsal 
valve. 
Dorsal valve gibbous in the middle; the centre occupied by a prominent 
mesial fold, from which the surface slopes abruptly to the lateral 
angles, becoming more op less conc&ve within the lateral and cardino- 
lateral margins. The mesial fold is marked along the centre by a single 
deep groove, which extends to the beak of the valve. 
Surface marked by fine concentric striae, and sometimes by strong 
imbricating folds. In well preserved surfaces the striae and undulations 
are bent backwards in the middle of the mesial sinus, indicating a 
mode of growth in the shell corresponding to the ginus in the mesial 
fold of the opposite valve. There are also slight indications of inter¬ 
rupted radiating striae. In the spe-cimen from the Hamilton group 
