LOWER HELDERBERG ROCKS. 
233 
jHerista bisulcata. 
Atrypa bisulcata : Vanuxed, Geol. Report of the Third District New-York, 1843, p. 112. 
Shell ovoid or elliptical, gibbous or subventricose. Ventral valve most 
gibbous towards the umbo, marked by a narrow mesial sinus which 
extends from the beak to the base of the shell, very gradually enlarging 
below : beak prominent, incurved. Dorsal valve very gibbous above,- 
marked by a narrow depression from near the beak to the base : beak 
short and closely incurved. 
Surface marked by fine concentric lines and stronger wrinkles of growth, 
and, under a lens, by fine longitudinal striae : substance of the shell 
apparently punctate. 
I place this species under the Genus Merista with some hesitation, on account 
of the apparently punctate character of the shell, while the form and general aspect 
is similar to others of the genus. 
Atrypa reticularis. 
Plate XLII. Fig. 1 a - r . 
For synonyms, references, etc., see Palaeontology of New-York, Vol.ii, p. 72. 
The figures represent the variety of form which occurs in the rocks of this group. 
Although there are perhaps no technical diiferences between this one and those of 
the Clinton and Niagara groups, it yet assumes a greater variety of form, ranging 
from subglobose to ovoid as in figures 1 a - k , and the dorsal valve becomes ex¬ 
tremely ventricose. 
In the Upper Pentamerus limestone this shell is more finely striated, as in fig. 
1 /, and more nearly circular in outline than those of the Shaly limestone below. 
The muscular area is proportionally larger in the specimens from this rock than 
in those from the Niagara group, as may be seen on comparison of figs. 1 n & 1 o 
below with fig. 5 s , Plate lv of Vol. ii. 
Fig. 1 a , b, c, d. Ventral and front views of young specimens. 
Fig. 1 f, g, h. Dorsal, front and profile views, showing the ordinary size and proportion of 
the larger individuals. 
Fig. 1 e. An individual of full size. 
Fig. 1 i, k, l. Dorsal, ventral and profile views of an elongate or ovoid form of this species. 
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