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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 trochitac have been obtained, are many of 

 them of the same colour, and evidently from an impregnation with 

 copper. 



Upon exposing the external parts of entrochi, &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, be- 

 sides 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 entrochius, 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 

 membraneous flocculae adhering to its sides, and on attempting to re- 

 move it, for the sake of obtaining a closer inspection of it, the bot- 

 tle, by accident, suffered a sudden jar, and immediately a fine trans- 

 parent membrane separated from about three-fourths of the inferior 

 surface of the trochite, and hung suspended from it in the fluid; bear- 

 ing 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 XI II. 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 



