GENUS MERXSTELLA. 
S95 
CrENUS MEMSTELEA* (Hall). 
This genus was described in the Thirteenth Report on the State 
Cabinet in 1860, for shells which had heretofore been included under 
the genus Athyris and subsequently under Merista. Differing from 
Athyris, they possess the general external form and characters of Merista, 
but have not the internal septum so characteristic of the latter genus.f 
* See note on page 297. 
t In discussing the relations of this group of shells with Athyris and Merista above cited, I 
have made the following observations : 
Among the fossils referred for many years to Terebratula, Atrypa, etc., European authors have 
separated the Genera Athyris and Merista ; shells which have many characters in common, and 
which were indeed at first united under Spirigera or Athyris, until in 1851 the Genus Merista was 
proposed by Prof. Suess. In my later studies of the Brachiopoda of the American palaeozoic strata, 
I have referred to tbe Genus Athyris certain species which have a subglobose or ovoid form, with 
lamellose surfaces, and without, or with scarcely perceptible radiating striae; while other forms, which 
are less distinctly lamelloscf and always more or less distinctly radiatingly striate with fine concentric 
lines of growth, I have referred to the Genus Merista. Many of the latter have the general form and 
surface characters of Merista (Atrypa ) tumida, Dalman, but are less ventricose : they all have inter¬ 
nal spires, and when perfect, the beaks appear to be imperforate. The radiating striae, though visible 
in well preserved specimens, are still more conspicuous in the partially exfoliated shell. Atrypa tumida 
of Dalman is cited by Davidson as one of the types of the Genns MerISta. 
I proposed last year ( Twelfth Report on the State Cabinet) a separation of certain Merista-like 
forms, under the name Camarium, on account of the presence of an arching transverse septum in the 
ventral valve. Subsequently, a more careful consideration of the characters of Merista as given by 
Mr. Davidson, and an inspection of his figures, have shown me that this arching septum, in its attenu¬ 
ation towards the beak, is identical with the shoelifter process described as belonging to the Genus 
Merista. An examination of numerous specimens of different species of those which I have referred 
to the Genus Merista, shows no evidence of this process or septum; and the deep muscular impres¬ 
sion below the rostral cavity, and the thickening of this part of the shell, are characters incompatible 
with the existence of the septum. Moreover I conceive that this arching septum, or the extension of 
the shoelifter process into the cavity of the valve, would produce such a modification of the soft parts 
of the animal that the inhabitants of these shells were generically distinct from those of the large 
uninterrupted cavity of the shells which I have heretofore referred to Merista. 
In order, if possible, to reach a solution of the question, I have had the shell removed from a solid 
specimen of M. tumida ( from Dudley, England), which is one of the types of the genus, and there is 
certainly no evidence of the septum or shoelifter process, but, on the contrary, the presence of all the 
characters marking the American species which I have referred to Merista in Yol. iii, Pal. New-York. 
At the same time, the Merista (Terebratula ) scalprum of Barrande [M. herculea of Barrande, or 
M. scalprum of Rcemer ], in the most solid of the specimens which I possess, readily reveals the 
presence of the septum. 
