LOWER HELDERBERG ROCKS. 247 



Merista Isevis. 



Plate XXXIX. Fig. 3 & 4. 

 Atrypa Itevia : Vancxem, Geol. Report of Third District, 1843, pa. 120, f. 2. 



Shell ovate, thin, somewhat ventricose. Ventral valve the larger, most 

 gibbous in the centre and umbonial region, having a shallow mesial 

 sinus extending from the front more than half way to the umbo : beak 

 prominent, ventricose, incurved, not perforate. Dorsal valve regularly 

 convex, gibbous in the middle, but without a defined mesial fold : beak 

 incurved. 

 Surface smooth, or marked by obscure concentric lines and occasional 

 stronger concentric wrinkles of growth, and, in the exfoliated shell, 

 by obscure radiating striaB. 



Some varieties of this species, especially the adult shell, often resemble Merista 

 bella, but differ in being proportionally longer, and are never marked by a sinus 

 on the dorsal valve. Young shells are sometimes comparatively much more com- 

 pressed, and often destitute of any trace of a sinus on either valve. 



This species presents considerable variety in form and proportions; but after 

 comparing extensive collections, I am compelled to unite all the varieties repre- 

 sented by figures 3 a - * as this species. There are numerous intermediate forms, 

 some of which have relations to those on the following plate; but the latter are 

 generally constant in the characters exhibited. 



The species was founded by Mr. Vanuxem upon specimens from the compact 

 portion of the shaly limestone of Herkimer county, which usually have a more 

 gibbous form than those of the Helderberg mountains. 



Fig. 3 a, b, c, d, e. Dorsal, ventral, profile, and front views of young individuals. 



Fig. Sf, g, h, t, k. Dorsal, profile, and front views of larger specimens. The specimen fig. 

 3 g presents no emargination in front, as do the specimens Sf-k. 



Fig. 3 /, m, n. Front, dorsal, and cardinal views of a full-grown individual. 



Fig. 3 0, p, r, s. Dorsal, profile, and ventral views of a larger specimen, showing a stronger 

 emargination in front, and a mesial sinus extending nearly to the beak of the 

 ventral valve. 



Fig. 3 t, u. Dorsal and profile views of a larger individual, which has no sinus or elevation 



upon the valves. ( It is possible that this may prove a distinct species). 

 Fig. 4 a. Interior of the ventral valve. 

 Fig. 4 6, c, d. Casts of the ventral valve, and profile of the same. 



The specimens figured on Plate xxxix, fig. 1 & 2, present some variations from 

 the prevailing characters of M. lavis; but I have not had an opportunity of seeing 



