W. J. S0LLAS ON PHARETROSPOtfGIA STRAHANT . 



243 



increase, the edge will undulate on each side of an imaginary median 

 plane, and thus the gentle folds a b c, of PI. XI. fig. 1, shown in the 

 diagram, Fig. 2, may have arisen. 



Fig. 1. — Section of 

 curve of Sponge. 



Fig. 3. — Section of 

 recurved fold. 



Fig. 2. — Section of 

 undulated form. 



Fig. 4. — Diagram of approxi- 

 mated sides of a fold. 



The folds so produced become more marked, and one or more may 

 increase so as to become recurved, as at PI. XI. fig. 1 c, here repre- 

 sented in profile (Fig. 3). 



The recurved fold, continuing its growth, may reach the surface 

 towards which it tends, and then, by union with it, form a tubular 

 process ; the latter, however, more usually arises from the union 

 of the sides of a fold which have grown towards each other, e. g. 

 PI. XI. fig. 1 e (and in the diagram, Fig. 4). 



The foregoing changes may be observed in most of the stages 

 described in one and the same specimen, e.g. that represented in 

 PI. XI. fig. 1 ; and thus the originally simple plate may develop 

 into a very complex whole ; and since the form, proximity, size, 

 and complexity of its foldings differ in different specimens, the 

 sponge well deserves to be described as " polymorphous." 



In most of the specimens from the Greensand the hollow tubular 

 processes and nearly closed folds are filled up with hardened chalk- 

 marl, which must be removed before one can analyze into its ele- 

 ments a form which at first sight appears to be almost inextricably 

 complicated. 



General Structure. — The plate is composed, as may readily be seen 

 on breaking it, of an irregular or vermiculate network, consisting 

 of white calcareous fibres imbedded in a solid matrix, either of har- 

 dened calcareous or compact phosphatic material. The ordinary 

 intermeshes of the network are not markedly modified either for the 

 outflowing canals or for their oscular openings ; hence the oscules 

 are small (the earlier writers on fossil sponges would have said 

 " absent "), and the excurrent canals are scarcely indicated in the 

 skeleton. 



Nevertheless there is a clear distinction between the two surfaces 

 of the sponge, accompanying a differentiation by which the one is 

 specially set apart as an oscular face and the other as originally a 

 poriferous one. 



e2 



