. 
SPONGITES IN CHALK AND FLINT. pA | 
_ states of mineralization in which sponges occur in the chalk 
give rise to many beautiful and highly instructive fossils, as 
we shall point out in the course of this notice. In general 
the zoophyte is simply invested by the flint, the pores and 
_ tubes being filled with silex, the original tissue appearing as 
_a brown reticulated calcareous mass, In other examples the 
sponge has been permeated by the liquid flint, and subse- 
quently perished; and in this manner have been formed 
those hollow nodules which on being broken are found to 
contain only a powder, consisting of siliceous spicules and 
fragments of silicified sponge. But in numerous instances 
_ the substance of the zoophyte is completely silicified, and 
_ the intimate structure of the original exquisitely preserved ; 
such are many of the flint-pebbles, and moss-agates, that 
are mounted as brooches and other ornaments. 
Sponeites.*“—This name I would apply generically to 
those fossils which appear to be identical in structure with 
_ the ordinary marine sponges that consist of a fibro-reticulated 
_ porous mass, destitute of regular tubes or canals: the form 
_ exceedingly various. 
The fossil sponges of the chalk may be divided into two 
_ groups; the cyathiforms, or cup-shaped, and the ramose, or 
branched. Flints inclosing the first kind, generally exhibit 
externally the form of the original; those containing the 
branched species are of irregular shapes, and except by an 
_ experienced observer, the nature of the enclosed body would 
not be suspected. On breaking them, the sponge is often 
well displayed, as in the specimen figured in Lign. 69, fig.2: 
_ the surface of this fossil was covered with a white gritty 
_ powder, made up of minute needle-shaped siliceous spicula. 
Sponeites Ramosus.—A branched sponge, sometimes from 
twelve to fifteen inches long, is not uncommon in the flints 
of the Lewes and Brighton chalk ; the stems and branches 
are cylindrical, and the terminations of the latter are rounded 
* Achilleum of Schweigger. 
