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not allow that a similar inference should be made in favour of those 
which are of a green colour ; although the recent Palmier Marin of 
M. Guettard was of a green colour ; since the other petrifactions from 
Plauen, whence the green trochitae have been obtained, are many of 
them of the same colour, and evidently from an impregnation with 
copper. 
Upon exposing the external parts of enti'ochi, &c. to the action of 
weak acids, I had repeatedly observed that, on handling them, they 
yielded a slippery feel ; and was thence led to conjecture that, 
besides the investing matter so evidently observable about the inferior 
extremities of encrinites, a membranous covering might have been spread 
externally over the whole of the vertebral column, limbs, &;c. of these 
animals. 
With the hope of obtaining some information on this subject, I 
therefore suspended an entrochus, consisting of two trochites similar 
to that which is depicted Plate XIII. Fig. 12, during twelve hours in 
vinegar. Upon examining this, it was found to be reduced to nearly half 
its size : the remaining fragment being perfectly smooth, and free from 
any adhering matter. Suspecting that the acid had acted with too 
much force, I suspended a similar entrochite, formed of two trochites, 
in two ounces of water, to which ten drops of muriatic acid had been 
added. On examining this, after twenty-four hours, I found several 
membranous floccul 0 e adhering to its sides, and on attempting to 
remove it, for the sake of obtaining a closer inspection of it, the 
bottle, by accident, suffered a sudden jar, and immediately a fine 
transparent membrane separated from about three-fourths of the inferior 
surface of the trochite, and hung suspended from it in the fluid ; 
bearing every mark of that surface, and most particularly of its crenated 
edge. A faithful representation of the appearance of this preparation, 
which was preserved two or three months, is given Plate XIII. Fig. 
47. Thus was afforded another instance of the curious fact of the 
extrication of an animal membrane from a mass of stone, in which it 
