UPPER, OR FLINTY CHALK. 163 



or 0'8 inch in diameter; and very rarely possess any vestiges of branches. The 

 present example was found in the centre of a large flint, from South street, 

 and is the finest hitherto discovered. It is of a compressed cyUndrical 

 form, nearly eight inches long, and exhibits the remains of seven branches ; 

 yet it is evidently only a fragment of a large specimen. Its structure is 

 spongeous, and it is destitute of an epidermis, or external covering. 



This spongite appears to have been of so delicate a texture, as not to 

 allow of its preservation in the chalk ; and it is only when enveloped in 

 siliceous nodules that any distinct traces of it remain. The constituent 

 substance is partly siHceous, and partly calcareous, the centre being 

 generally composed of flint, and the external spongeous mass, of a friable 

 white or yellowish carbonate of Ume. In a few instances the surface is 

 frosted over with drusy crystals of quartz. 



The specimens are for the most part either loose in the cavities of the 

 flints, or but very slightly adherent; a circumstance that may have ori- 

 ginated from the decay of the epidermis of the original, or from a con- 

 traction of its substance. 



As this species of fossil sponge is very common, it seemed desirable to 

 distinguish it by some appropriate name ; that of S. ramosa has therefore 

 been assumed as a temporary distinction. 



Tab. XV. fig. 8, is the sihceous cast, or nucleus, of a fossil of this kind, 

 the friable spongeous mass having been removed. Similar specimens are 

 not unusual in the cavities of those nodules in which the original zoophyte 

 has suffered decomposition, subsequently to its immersion in the flint. 



Localities. In every quarry in the upper chalk near Lewes, and 

 Brighton. 



12. Branched silicified zoophytes, belonging to some unknown genus. 



It is scarcely possible either by description or delineation, to convey an 

 accurate idea of these curious fossils. They generally occur in the centre 

 of the largest flints, and are more or less ramose. Some specimens have 

 from four to six branches, the terminations of which appear on the surface 

 of the nodule, in the form of annular depressions. Upon fracture, these 

 bodies are found to consist of innumerable diverging tubuli, proceeding 



y2 



