18/2.] DR. J. S. BOWERBANK ON THE SPONGIAD^E. 125 



This sponge is remarkable for being the living type of a well-known 

 flint fossil from the chalk, named, figured, and described by Dr. 

 Mantell, in his ' Fossils of the South Downs, or Geology of Sussex,' 

 p. 179, pi. 16. figs. 19, 20, and 21, as Choanites Kamigii. The 

 author appears to have read off from his fossil specimens very cor- 

 rectly the former history of the structural characters of the sponge ; 

 and he has described its radical processes as they appear in numerous 

 specimens of the fossils as well as in the recent sponge. The follow- 

 ing is the specific character he assigns to the fossil. 



" Inversely conical, externally marked with irregular fibres, some 

 of which penetrate the substance and terminate in openings on the 

 inner surface ; central cavity cylindrical, deep, narrow; base fixed 

 by radical processes." 



With very slight alteration this description of the fossil would 

 answer as nearly as possible for a specific description of the recent 

 specimen. 



I have several fine specimens of this fossil— one, a natural longi- 

 tudinal section through the middle of the sponge, exhibiting the 

 surface of the great cloacal cavity and the numerous fossilized canals 

 radiating from it. Their entrances into the cloaca are covered by a 

 thin layer of silex. In another specimen, which has no extraneous 

 flint around it, the fibrous structure is indistinctly visible on the 

 exterior of the mass ; but on making a longitudinal section of it the 

 fibres all round the distal end of the cloaca became beautifully 

 visible, every fibre being covered by a thin layer of silex, while the 

 interspaces were entirely free from that material. In a third speci- 

 men, two of these sponges have coalesced and become as one, follow- 

 ing exactly the law that always obtains with recent sponges under 

 such circumstances. On some parts of the external surface there 

 appear doubtful traces of the remains of the dermal integument • and 

 over all the other parts the interlacing skeleton-fibres slightly coated 

 with silex are visible by the aid of an inch lens. I have never seen 

 a specimen of this fossil which exhibited the fossilized dermal inte- 

 gument, excepting in a thin transverse section of one from apparently 

 about the middle. It is a very regular oval of 3 inches by 2^ inches ; 

 and in this the dermal integument is indisputably present. Unfor- 

 tunately I could obtain no more of this beautifully illustrative speci- 

 men than the slice in my possession. 



I have entered thus minutely into the comparison of the fossil 

 with the recent sponge, as it appears to afford strong additional 

 evidence of the great antiquity of the land- and water-productions 

 of the southern portions of our globe still above the sea-level ; and 

 it appears, from the similarity of the recent productions of that 

 portion of our earth, that they are very closely allied to the fossil 

 productions of the chalk period of our part of the world. The same 

 course of reasoning may be applied to the Loudon-Clay period; 

 among the fruits and seeds of that formation their nearest allies were 

 almost invariably to h« found among the fruits and seeds of our 

 Australian colonies. 



