246 Prof. Ansted on the Zoological condition of Chalk Flints, 6^c. 



near Norwich, and are generally insulated in the chalk, sometimes 

 lying horizontal and sometimes inclined or erect, but although 

 silicified, not apparently connected wath the layers of flint. These 

 singular fossils are cylindrical, fusiform or cup-shaped, and they 

 are occasionally found planted as it were one above another, the 

 upper one being closed at the toj) and attached to the open lip of 

 that immediately below. They all have a hollow open axis filled 

 with chalk, and a central tube about the thickness of a finger, and 

 consisting of siliceous particles, is traceable through the chalk 

 from the base to the vertex*. All these three genera were doubt- 

 less affixed by radicles to solid rock and possessed no powers of 

 locomotion. 



Lastly, the Polypothecia represented the branching sponges as 

 the former genera did the large cup-shaped sponges of the ex- 

 isting seas. They are frequently found inclosed in flint, the Po- 

 lypothecia having in this case been partly surrounded by another 

 sponge and silicification taking place in the whole mass together. 

 The branching sponge however not decomposing at the same rate 

 as the other, we often find its remains represented by a loose cast 

 in the substance of a common flint. 



It appears then, that in the zoological condition of those flints 

 which ofi"er external proof of their organic origin as well as in 

 the microscopic structure of the others, which are regularly stra- 

 tified in tabular masses, the principal, if not the only, accumula- 

 tions of siliceous matter in the chalk are upon sponges ; that sin- 

 gvdar class of organic beings, the very fact of whose possessing 

 life has sometimes been doubted. It only now remains to con- 

 sider how far the external relations of such fossils wth the chalk 

 itself renders it probable or otherwise that all chalk flints are of 

 spongeous origin. 



In those cases in which the flint is perfectly tabular, these re- 

 lations can of course have no further interest than that which 

 arises from the condition of the surface of the flint which we have 

 already considered ; but it is not an uncommon accident that por- 

 tions of some organic bodies should adhere to or be imbedded in 

 the flint, sometimes projecting from it to a considerable distance 

 and ofiering strange and grotesque forms. Such appearances are 

 easily explicable on the h}^3othesis of sponges growing at the bot- 

 tom of an ocean, in which from time to time various fragments 

 of shells, &c. were deposited and partly inclosed by the sponges ; 

 and it is not at all necessary to assume that these fragments were 



* The organic nature of the siliceous matter in the Parajnoiidra is suffi- 

 ciently clear from the evidence of Prof. Ehrenberg on this subject. He states, 

 that although lie failed to discover in them the structure of well-preserved 

 sponges, he could perceive the contorted remains of decomposed sponges 

 along with the remains of Infusoria. — Annals of Nat. Hist, vol. ii. p. 162. 



