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 Camabium, 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 sequently the name was withdrawn.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 Mekistella, 

 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. Roemer 

 ( = A/, plebeia, Sowerby), from the Eifel and Devonshire. A careful examina- 

 tion of a considerable number of individuals from Pelm shows that a " shoe- 

 lifter " is quite as conspicuously developed in the brachial 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 doobt that this plate in Merista is quite analog'ous to the supported spondylium of 

 Pb>tambbd8, Cahabophoria, etc., as well as to the platform of the Trimerellids, to which attention has 

 been directed in the pi-ece<iing volume of this woik. Of all these forms Mbrista i.s the only one in which this 

 pUle or platform is not supported by a median septum, though, as noticed lielow, such a supporting septum 

 exists in certain Devonian nioristnids. For the unsupported triangular plate, occurring in the pedicle-valve 

 of the genera Adlacorhymcuds and Eicuwaldia, it may be necessary to find a diffei-ent interpretation, as 

 ■oggMted in a subsequent chapter. 



