Mr. T. Austin on the genus Dunstervillia. 407 



sponge [Dunstervillia), discovered on the coast of Southern Africa 

 by Mr. G. Dunstervillc, I was at once struck with the resem- 

 blance this minute sponge bears to the SpharoniteSf even before 

 I had arrived at that part of Mr. Bowerbank^s communication in 

 which he. states his conviction that the fossil known as Sphcsro- 

 nites tessellatus owes its origin to an allied genus of Zoophytes. 

 And I also came to the conclusion that this minute sponge would 

 throw considerable light on those hitherto puzzling fossils, the 

 Ischadites of the Silurian system. 



On referring to the illustrations in the ' Annals/ and compa- 

 ring them with Mr. Bowerbank^s very clear description, there can 

 be no room to doubt the correctness of his opinion that the Sphce- 

 ronites tessellatus is the calcareous skeleton of a spongiform body. 

 And if it is admitted that S. tessellatus is the remains of an ex- 

 tinct species of sponge, there can be but little difficulty in proving 

 the Ischadites to be of similar origin, and not belonging to the 

 family of Ascidice, as Mr. Konig imagined. In alluding to this 

 fossil in Murchison^s ' Silurian System,^ Mr. Konig says, " they 

 seem to form a group of globular, coriaceous, and it may be added, 

 pedicled bodies, for in one of them a cicatrix for the insertion of 

 the pedicle distinctly appears.^^ This cicatrix is probably the 

 point where the zoophyte had been attached. 



The manner in which the Ischadites are found associated to- 

 gether, and are compared in Murchison^s ' Silurian System ^ to 

 compressed figs, serves to show that the zoophytes were affixed 

 in groups to extraneous bodies, and that they lived and died on 

 the spots where their remains are now imbedded. The flattened 

 form in which they sometimes occur may be accounted for by the 

 fact that the internal supports (spicula) are either wholly want- 

 ing, or, if present, but little adapted to sustain the sponge in its 

 original form after the destruction of its vitality, so that the cal- 

 careous framework which still held together would become col- 

 lapsed and leave the remains in the shape we now find them. 



The Sphceronites have been heretofore frequently considered as 

 allied to the Crinoidea, but Mr. Bowerbank has, I conceive, clearly 

 removed the difficulty relative to this fossil, and which will hence- 

 forth take its proper position in all future scientific arrangements 

 of organic remains. 



I will now venture an opinion relative to two other fossils which 

 have caused considerable diversity of ideas respecting their nature 

 and origin, namely, the Tentaculites and Conularia. 



On a careful examination of numerous specimens, I am of opi- 

 nion that the Tentaculite is the shell of a Pteropodous mollusk 

 allied to the recent Creseis, as the Conularia is that of an animal 

 allied to the Cleodorce. 



Bristol, May 14, 1845. 



