9S0 PALEONTOLOGY OF NEW-YORK. 



Some varieties of this species bear considerable resemblance to M. lavis; but 

 they are always proportionally broader, the hinge line declining less rapidly; the 

 umbo of the ventral valve is less gibbous, while it is more regularly arcuate and 

 broadly sinuate in front. The dorsal valve is very gibbous along the centre, some- 

 times forming an indistinct ridge, while the sides slope more abruptly to the 

 margins which are often compressed. 



 



Fig. 1 a, b. Dorsal and cardinal views of a young specimen. 



Fig. 1 c, d, e,f. Dorsal, ventral and profile views of large specimens. 



Fig. 1 g, h, I, k, I. Dorsal, ventral, profile and front views of full-grown individuals, show- 

 ing the forms of well-preserved specimens. 



Fig. 1 m, n. Dorsal and profile views of a very gibbous form, which is unusually extended 

 laterally. 



Fig. 1 o. Interior of a ventral valve. 



Fig. 1 p. Cast of the beak of the ventral valve. 



Fig. 1 t. An imperfect specimen which has been eroded at the sides, showing the internal 

 spires. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains; Schoharie, Carlisle, Catskill, Hudson, and other 

 places. 



CASTS OF MERISTA : PROBABLY OF M. arcuata. 

 Plate XLL Fig. 2 a -g-. 



Fig. 2 a. Cast of the dorsal valve. 



Fig. 2 b. Cast of the ventral valve, showing the muscular impressions and extended process 

 which filled the beak of the shell. 



Fig. 2 c, d. Casts of the ventral valve, showing some diflferencos in the form of the muscular 

 impressions, and the portion filling the cavity of the beak. The radiating striae 

 are partially preserved on fig. 2 c. 



Fig. 2 c. Cardinal view of a well-preserved cast. 



Fig. 2 f, g. Profile views of 2 rf and 2 c respectively ; the specimens 2 c, g, preserving the 

 cast of both valves, 2 d,f being the ventral valve only. 



These specimens are usually much compressed, and a large proportion of those 

 found are distorted. The figures illustrate very satisfactorily the characters of the 

 interior; and the marked difference between these and the casts of Jlthyris or 

 Spirigera, as occurring in our strata, is very conclusive of the generic importance 

 of Merista. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Becraft's mountain, Hudson. 



