Vol. 56.] FROM THE EOCENE OF VICTORIA. 65 



Tretocalia pezica. gen. et sp. nov. 

 Figs. 20-26. Specimens of various forms, showing their mode of growth. 



Natural size. From Eocene beds at Flinders (Victoria). Collected by 



Mr. T. S. Hall. 

 Fig. 27. A portion of a transverse section, showing the fibres, the direction of 



the canals, and the thickness of the cloacal wall of the sponge, x 10. 



28. Some of the skeletal fibres in vertical section, showing indistinctly the 



disposition of the spicular rays. X 60. 



29. Some of the skeletal fibres as seen in transverse section. X 60. 



Plate V. 



Bacironella parvula, sp. nov. 



Figs. 1-3. Specimens showing the various modes of growth. Fig. 2 is encrusting 



the upper portion of a sea-urchin spine. XlO. From Eocene beds 



at Mount Martha, Mornington (Victoria). Discovered by Mr. B. 



W. Priest. 

 Fig. 4. A vertical median section of a specimen which has grown over and 



completely enclosed a fragment of polyzoon. x 10. 

 Figs. 5-8. Three- and four-rayed spicules from the basal layer. X200. 



9-14. Irregularly modified four-rayed spicules which connect transversely 



the skeletal-mesh spicules and also form the basal layer of the sponge. 



X200. 

 Fig. 15. A portion of the skeletal mesh, showing the arrangement and mode 



of junction of the component spicules. X200. 



16. Part of the outer surface of the sponge, showing the spicules bounding 



the radial canals. X 100. 



17. A fragment of the skeletal mesh, near the outer surface of the sponge. 



X100. 

 Figs. 18 & 19. Portions of the mesh next the surface, showing a calcific tuber- 



culatecl pellicle. X 100. 

 Fig. 20. An imperfect skeletal spicule; one of the rays is strongly hollowed out 



for clasping. X 200. 



21. A fragment of the close mesh of the basal layer, X200. 



22. Four-rayed spicules of the basal layer. X 200. 



Discussion. 



Prof. Sollas expressed his sense of the value of this important 

 paper. The pseudo-Lithistid character of the skeleton of Petrostroma 

 was a fact of great interest, which was deepened by the Author's 

 discovery of allied sponges, differing in details but still distinguished 

 by the same character. As regards the chamber-system, it had 

 already been shown that this had passed through similar stages of 

 evolution in the calcareous and siliceous sponges. The Lithonina 

 now proved that a similar homoplasy is presented by the skeleton 

 in the two groups. Two mechanical problems — one the economiza- 

 tion of energy in the production of water-currents, and the other 

 the best disposition of rigid material to resist stresses in the 

 organism — had been solved in practically the same manner and 

 independently, by two different groups of organisms. The bearing 

 of this fact on Darwin's theory of evolution was worthy of close 

 consideration. It was reassuring to learn that, notwithstanding the 

 general resemblance of the Lithonine and Lithistid skeletons, there 

 was no insuperable difficulty in distinguishing them by their morpho- 

 logical characters even in the fossil state. A fresh beginning seemed 



Q. J. G. S. No. 221. p 



