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with any animal substance now existing; indeed, so considerable is 
that difference, that some substances will be placed before you, which 
not only cannot be referred to any particular known species, but 
which would almost authorize the formation of new genera for their 
reception. 
We shall proceed, however, as nearly as possible, according to the 
generally accepted systematic classification ; and shall derive what 
aid can be obtained, from the examinations which have been made of 
living substances apparently of a similar nature. It is intended, there- 
fore, to endeavour to comprise, under the genus alcyonium or spongia, 
the substances so accurately inquired into by M. Guettard, as well as 
several others which have not been spoken of by him, but are evidently 
of the same kind. 
With respect to the classification of these bodies, a difficulty almost 
insuperable presents itself ; since the characteristic marks by which 
the substances belonging to these two genera are distinguished, in a 
recent state, are frequently not to be discovered after they have sus- 
tained the change of petrifaction. Previously, however, to proceeding 
further in an inquiry on this subject, it will be proper to consider the 
nature of both alcyonium and of sponge, and to ascertain what are the 
distinctive characters of each. 
The alcyonium is an animal which assumes a vegetable form, and 
which is either of a fleshy, gelatinous, spongy, or leathery substance, 
having an outward skin full of cells, with openings possessed by oviparous 
tentaculated hydra : the whole substance being fixed to some other body 
by a seeming trunk or root. 
Count Marsilli, who carefully examined not only the physical, but 
the chemical properties of these bodies, observes that they are all 
surrounded by a porous leather-like bark : and that the interior sub- 
stance is, in some, a jelly-like matter, and in others, a mass of light ash 
coloured acicular spines, which prick the hands on being handled, in the 
same manner as do the spines of the plant called the Indian fig. 
