Plate 18. 



Fig. 1. Gellius perforaius. Upper surface; from a photograph ; \. 



" 2. Oceanapia hacilUfera. Fistula ; from a photograph ; \. 



" 3. Hymeraphia, sp. incrusting Oceanapia bacillifera. Spicules ; x 450. 



" 4. Oceanapia bacillifera. Tangential section through wall of fistula, showing 



arrangement of skeletal spicules and spongin. Meshes 

 in the skeletal framework, which in the figure are 

 represented as vacant, are filled with parenchyma; x 70. 



" 5. Tylodesma alba. Megascleres ; x 70. 

 6. " " Sigmata ; x 450. 



" 7. " " From a photograph. Sponge viewed obliquely from the side, 



so that upper surface is seen ; \. 



" 8. Tylodesma vestibularis. Megascleres ; x 70. 



" 9. " " Sigmata ; x 450. 



" 10. Fhakellia lamelliijera. Outer surface of skeletal framework. Piece has been 



macerated in potash. From a photograph ; x 5. 



