228 Prof. P. M. Duncan on the Genus Hlndia. 



which alone led me to the expression of the belief in the 

 original calcareous condition of the skeleton ; it was the dis- 

 covery of a penetrating-, parasitic, unicellular, vegetable 

 organism within the canals and traversing the spicules which 

 led mainly to the belief. Dr. RaufF mentions this Palceachlya, 

 and notices correctly that it influenced my opinion that the 

 skeleton was not siliceous in the living state. 



But whilst he came to satisfactory conclusions regarding 

 Prof. Steinmann in a perfectly scientific manner, my fellow- 

 labpurer considered the PaJceachlya a qiiantite neglige ahle. 

 ^^ \ venture to refer any body who may take an interest in 

 this discussion to read the papers on the subject of the perfo- 

 rating parasitic Thallophytes recent and fossil (Proc. Poyal 

 Soc. 1876, no. 174, p. 238; and Quart. Journ. Geol. Soc. 

 1876, p. 205) ; also a communication to the Royal Micro- 

 scopical Society, 1881, on the cavities within siliceous sponge- 

 spicules, the result of vegetable organisms (Journ. Royal 

 Microsc. Soc. ser. 2, vol. i. p. 557). y 



No long tubular vegetable structures with organs of repro- 

 duction have ever been found ramifying in siliceous skeletons, 

 and the resemblance of the parasitic organisms of the Silurian, 

 Devonian, and subsequent geological ages (found in calca- 

 reous fossils) to those in the shells of MoUusca, Corals, and 

 Foraminifera of the present day is most remarkable. The 

 penetrating Thallophytes of the present day belong to the 

 same group as the ancient ones, and they are and were depen- 

 dent upon the organic matter (connective tissue) which is 

 within the calcareous structures of Mollusca, Corals, &c. 



I maintain that the Pala'acldga grew and lived in the sponge 

 as it did in the corals of the same age, and that it was not intro- 

 duced after fossilization. It was the presence of these tubular 

 forms of many sizes within the calcareous element, as well as 

 free in the canals of the Hincliaj that made me believe the 

 original skeleton was calcareous, not, as Dr. Hinde puts it, 

 because the calcareous element now exists. 



Fully appreciating Dr. Hinde's excellent work, and acknow- 

 ledging the force of the arguments he has adduced to prove 

 the occurrence of calcite after silica, I nevertheless must con- 

 sider the argument I have brought forward to be of importance. 

 Of course the statement that the mimetic series of calcareous 

 sponges once existed, is within reasonable distance of the 

 tiuth, for who amongst us is to limit Nature as regards possi- 

 bilities ? (Specimens of Hindia sphceroidalisy nobis, are now 

 in the British Museum, and are portions of the type.) 



