PALAEONTOLOGY OP NEW-YORK. 
The shells of this genus are oval, ovoid or suborbicular, elongate or 
rarely transverse; valves unequally convex, with or without a median 
fold or sinus; beak of the ventral valve often with a circular fora¬ 
men, and incurved over the umbo of the dorsal valve. Area none; valves 
articulating by teeth and sockets ; surface smooth, or with fine concen¬ 
tric lines of growth, and with very fine, indistinct or obsolete, radiating 
striae. 
Since this shoelifter process, or septum, was originally described by Prof. Suess as characteristic 
of his Genus Merista, and the species designated by him as the types of this genus (the M. scalprum, 
M.herculea of Barrande ) “do possess this feature, the genus must he retained for the species 
** with the shoelifter process.” 
It would appear, therefore, that the Genus Camarium, proposed by me in the preceding Report, 
possesses characters identical with Merista as originally described by Suess, but which have been 
overlooked to some extent in consequence of the reference to M. tumida as a typical form of the genus. 
* * * * # At the same time, as the M. tumida of Dalman, an English and Swedish species, in 
common with numerous well-marked forms in our Silurian and Devonian strata, do not possess this 
feature, we can no longer, with propriety, refer them to that genus. 
With this restriction, the Meristas proper consist of smooth, ovoid, circular or transverse shells, 
with usually a conspicuous sinus upon the ventral valve, and a corresponding wide, often undefined, 
mesial fold or elevation upon the dorsal valve; the hinge articulation being not very different from that 
of Athyris, to which they are allied. 
The interior of the ventral valve, however, is strongly distinctive; and the septum or shoelifter 
process is not unfrequently shown in the cleavage of the beak of that valve, in solid specimens, where 
the interior is inaccessible. 
The forms which I have regarded as Merista are similar to those above; but instead of this sep¬ 
tum, or shoelifter process, they have a deeply marked triangular muscular area just below the rostral 
cavity of the ventral valve, which is bordered on the anterior side by a callosity of the shell, and on 
the two other sides by the strong dental lamellae. This feature is not conspicuous in Athyris : the 
dental lamellae in that genus are shorter and less strong, and the form of the muscular impression is 
different. The dorsal valve of those shells now under consideration has a longitudinal median septum j 
a feature which is obsolete, or partially obselete, in the species of Athyris. In the Camarium, or 
Merista proper, the exterior of the ventral valve sometimes shows what appear to be two diverging 
septa, somewhat similar to those in the dorsal valve of Pentamerus, which are the margins of the 
shoelifter process. 
The Meristidse begin their existence, so far as we know, in the rocks of the Clinton group; and in 
this and the Niagara group there are several species, while they are more numerous in the Lower 
Helderberg group : they occur likewise in the Upper Helderberg rocks, and in the Hamilton group. 
Merista proper, so far as we know, appears first in the Lower Helderberg period, while Athyris is 
known in two species for the first time in the Hamilton group [also in Upper Helderberg group]. 
Restricting, therefore, the signification of the Genus Merista to such forms as were originally 
included by Prof. Suess under that name, it becomes necessary to designate those species of similar 
form, but without the peculiar appendage of the ventral valve, by another generic term; and I would 
therefore suggest the name Meristella, proposed by me last year.J 
t Twelfth Report on the State Cabinet, p. 78. 
