85 





 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 evi- 

 dently 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, ia 

 the same manner as do the spines of the plant called the Indian fig. 



