49 



The richest collection which is known of these fossils is, I 

 believe, that of the gravel pits of the iron sand at Farringdon, 

 where they are found mingled with the fossils of some of 

 the early formations. Some of these specimens are of con- 

 siderable size, and are in such excellent preservation as to 

 allow, at least, of their arrangement under the more compre- 

 hensive divisions of the genus. Among the specimens are 

 round and cavernous, resembling sp. globosa ; pediculated, 

 sub-pediculated, lobated and flabelliform, ramose, foliaceous, 

 cyathiform, funnel and ficiform. In most of the specimens 

 the structure and form of the substance are so obvious, as to 

 raise the hope that the determining of their specific cha- 

 racters, and of their consequent arrangement by some 

 zealous investigator of these interesting relics, may be 

 expected. 



Fossil remains of this substance are frequently found 

 in the chalk, and most numerously perhaps in the lower 

 series of this formation ; but from the delicate texture of 

 the membrane of the sponge, and from its cavities being 

 filled by the chalk itself, there are no known means by 

 which the fossil can be extricated from its matrix. Indeed, 

 it frequently happens that these substances, agreeing as to 

 colour with the chalk, would exist there undetected, but 

 by the greater degree of hardness which it possesses, and 

 by the asperities which it presents at the surface. They 

 are sometimes pointed out by the strong tint which they 

 derive from having sustained a ferruginous impregnation ; 

 and in these instances, although a sufficient separation from 

 the surrounding chalk to allow the developement of the 

 form cannot be obtained, yet, by their being carefully 

 rubbed down to a smooth surface, something of this may 

 be discovered, with, generally, a tolerable display of the 

 internal structure. 



The nodules of chalk flint frequently contain the sili- 

 cified remains of sponge, and in a state which will allow 



H. 



