446 F. A. Bather-^Groioth of Cephalopod Shells. 



had been replaced by silica on the transmutation theory, they would 

 not thus be dissolved out. I fully admit that the substitution of 

 silica for calcite is of common occurrence, but in the case of these 

 Carboniferous Cherty limestones it does not seem to have taken place 

 to any great extent. 



The main argument brought forward by Prof. Hull,^ in his lately 

 published pajDcr, against the derivation of the Chert from sponge- 

 remains, is stated to be " based on the fact that the development of 

 sponge-life in the Carboniferous period was insignificant, and quite 

 inadequate to account for the existence of bands and masses of Chert, 

 sometimes constituting almost a half or third of the entire mass of 

 the Upper Limestone." 



But in what manner did Prof. Hull set himself to ascertain the 

 fact that the Sponge-life in the Carboniferous seas was insignifi- 

 cant? With great pains he compiles lists of the numbers of species 

 and genera of sponges and calcareous organisms in the Carboniferous 

 and Cretaceous formations respectively, and he finds an " enormous 

 proportion of siliceous sponges in the Cretaceous as compared with 

 those in the Carboniferous period." Further, in the Carboniferous 

 period, " the species of siliceous sponges might almost be counted 

 •on the fingers of the two hands ; both in genera, species and indi- 

 viduals they are quite unimportant as compared with the calcareous 

 organisms of that period, and totally inadequate to supply materials 

 for the formation of such beds of Chert as are formed in the Carbon- 

 iferous Limestone formation." 



It seemed to me, however, that this method was rather an unsafe 

 one to determine a question of this character, and that even in the 

 hands of a Director of the Geological Survey, in his own field of 

 investigation, a satisfactory result would not be obtained by figuring 

 up lists of species, and then doing a rule-of- three sum with the totals. 

 I ventured to think that it would be more satisfactory to ask the 

 question whether sponge-life was insignificant in the Carboniferous 

 ■ period, of Nature herself, and so, with the limited time and resources 

 at my disposal, I went to the rocks in Ireland and collected the 

 evidence which has been brought forward in this paper. I leave it 

 to the reader to determine whether my plan, or that of the Director 

 of the Irish Survey, is most to be relied on, and whether, as Prof. 

 Hull states, there is absolutely no evidence beyond a fanciful analogy 

 for the organic origin of the Carboniferous Chert of Ireland. 



III. — The Growth of Cephalopod Shells. 

 By F. A. Bather, B.A., F.G.S., 

 of the British Museum (Natural History). 



THE shells of Cephalopods form an evolutionary series, traceable 

 to an ancestor unknown, but nearly related to not yet coiled 

 forms of the Cambrian. In all descendants from this common stock 

 homologous parts are discerned. Hence facts regarding the grovs^th 

 of any one form may be applied to all. Of Cephalopods with 

 chambered shells only Nautilus, Spirula, and Sepia exist. Spirula is 

 1 Proc. Eoyal Soc. 1887, vol. 42, p. 306. 



