72 
PALEONTOLOGY OF NEW YORK. 
the causes producing it, the cavity beneath it unquestionably enclosed and pro¬ 
tected delicate portions of the viscera."*" 
The term Camarium, Hall, was proposed for Merista typa (= Camarium typum), 
Hall, before the structure of M. herculea, Barrande, was well understood ; sub¬ 
sequently the name was withdrawn. Camarium typum is, however, a shell with 
some interesting peculiarities and susceptible of great variation in the form 
and size^of its “shoe-lifter.” This is sometimes very narrow, as in the other 
species of the genus, but is oftener very wide on the margin and may extend 
for fully two-thirds the diameter of the valve. Usually it is evenly and highly 
arched, but often is sharply angled and abruptly elevated. The dental lamellae 
may extend for a short distance over the surface of the plate, ending abruptly, 
or they may be produced along its margins as two greatly thickened, callous 
ridges. In these features, however, there does not appear to be any good basis 
for a separation of this species from its allies. 
The genus Merista has usually been regarded as ranging from the faunas 
of the upper Silurian (Wenlock, Etage E, etc.) into the middle Devonian. 
In European faunas it appeared before the age of the genus Meristella, 
but in America the appearance of the two genera was contemporaneous. It 
would be altogether natural to presume that species occurring so late as the 
middle Devonian and after so great an interval from the disappearance of the 
typical forms of the genus, must have undergone some more or less substantial 
modification. This is the case with the Devonian Merista scalprum, F. Boemer 
(= M. plebeia, Sowerby), from the Eifel and Devonshire. A careful examina¬ 
tion of a considerable number of individuals from Pehn shows that a “ shoe- 
lifter ” is quite as conspicuously developed in the brachiM as in the pedicle- 
valve, while the cavity beneath it is divided into two compartments by the 
median septum which extends beyond the anterior edge of the platform thus 
* There can be no doubt that this plate in Merista is quite analogous to the supported spondylium of 
Pbntambrus, Camarophoria, etc., as well as to the platform of the Trimerellids, to which attention has 
been directed in the preceding volume of this work. Of all these forms Merista is the only one in which this 
plate or platform is not supported by a median septum, though, as noticed below, such a suxiporting sejitum 
exists in certain Devonian meristoids. For the unsuxiported triangular xdate, occurring in the pedicle-valve 
of the genera AuLACORiiYNCHns and Eichwaldia, it may be necessary to find a different interpretation, as 
suggested in a subsequent chapter. 
