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PALAEONTOLOGY 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, Jc, 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 slaaly limestone of the Lower Helderberg 
group : Helderberg mountains; Schoharie, Carlisle, Catskill, Hudson, and other 
places. 
CASTS OF MERISTA : PROBABLY OF M. arcuata. 
Plate XLI. 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 ^alve, showing some differences in the form of tlie muscular 
impressions, and the portion filling the cavity of the beak. The radiating strim 
are partially preserved on fig. 2 c. 
Fig. 2 e. Cardinal view of a well-preserved cast. • 
Fig. 2 f, g. Profile views of 2 d 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 Athyris 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. 
