MERISTELLA OF THE UPPER HELDERBERG GROUP. 309 



Meristella anisalcata. 



SUBGENUS ? PENTAGONIA (Cozzehs). 



PLATE L. 



Atrypa unitulcata: Conrad, Annual Report on the Palieontology of New-Tork, p. 56. 1841. 

 Rhynchonella unitulcata : (Conrad, sp.) Hall in Tenth Report on the State Cabinet, p. 125. 1857. 

 jStrypa unirulcata : referred with doubt to Meeistella, and name Gonioccelia proposed, in 



Fourteenth Report on the State Cabinet, p. 101. 1861. 

 Compare Pentagonia peersii, Cozzebs, Annals of the New-York Lyceum, Vol. iii, pa. 158, pi. x, f. 2 a, 6. 



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 or less concave 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 striaa, 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 sinus in the mesial 

 fold of the opposite valve. There are also slight indications of inter- 

 rupted radiating striae. In the specimen from the Hamilton group 



