LOWER HELDERBERG ROCKS. 833 



Merista bisulcata. 



Mrypa bisukata : 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 stries : 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. 



Atrjpa 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 differences 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 thaii 



in those from the Niagara group, as may be seen on comparison of figs. 1 n & 1 o 



below with fig. 5 *, Plate lv of Vol. ii. 



Fig. 1 a, b, c, d. Ventral and front views of young specimens. 



Fig. 1 /, 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 «', k, I. Dorsal, ventral and profile views of an elongate or ovoid form of this specie^: 



32* 



