392 Scientific Proceedings, Royal Dublin Society. 



DESCEIPTION OF PLATE XV. 



Fig. 1. — Colossal acerate spicule of Leueonia glomerosa (?) (Bwk.), showing faces of 

 rhombohedra, revealed by erosion ; from a portion near the point. 



Fig. 2. — Similar, showing the eroded pointed end of the spicule. 



Fig. 3. — Two sagittal spicules from the same sponge, showing the eroded paired 

 rays, which are both striated and margined by rhombohedral faces ; and the unpaired 

 ray which presents no signs of erosion. 



Fig. 4. — Part of an eroded acerate, showing striae on one side, and rhombohedral 

 angles on the other. On one of these angles a rhombohedron has been indicated by 

 dotted lines, to show the relation of the striae and the angles, and the probable position 

 of the rhombohedron relative to the spicule. The apical angles of the rhombohedron 

 have been joined by a dotted line, o a, which, prolonged in both directions, indicates 

 the position of the optic axis. This makes an angle of about 72° with the edge of the 

 spicule, which should therefore extinguish at 20°. The striae make an angle of about 

 65°, and should extinguish at 7°. This approximates to an observation made, where the 

 striae made an angle of 66° with the edge of the spicule, which extinguished at 15°, the 

 striffi at 9°. 



Fig. 5. — Part of a colossal acerate, showing etch striae. 



Fig. 6. — Transverse section of a colossal acerate, which had been slightly etched by 

 acetic acid. 



■ Fig. 7. — A group of etch striae. 



Figs. 8 and 8a. — Two of the coronal acerates of Grantia ciliata, after heating nearly 

 to redness, showing broken edges, bounded by cleavage planes. 



Fig. 9. — Similar, showing the navicula-like bodies into which the spicule breaks up. 



Fig. 10. — A single one of these rhombs. 



Fig. 11. — A colossal acerate, after etching with acetic acid, showing groups of striae, 

 and on the right-hand edge below three sides of a rhombohedral pit. On the right-hand 

 side above is a triradiate marking, corresponding to the edges b, d, and d' in the projec- 

 tion, fig. 7, woodcut, p. 388. 



Fig. 12. — Outlines of transverse sections of shafts of colossal spicules in a Phare- 

 trone [Catagma porcatum) . 



Fig. 13. — Part of transverse section through the 'neck' of Grantia sp. from Van- 

 couver, N. America, showing arcs of nine out of sixteen circular rows of acerate spicides 

 visible in the preparation. 



Fig. 14. — Transverse sections through large acerate spicules of the same sponge. 



Fig. 15. — Transverse sections of acerate spicules shown in fig. 13 more highly mag- 

 nified : («) sections at right angles to the lines of sight ; 'h) slightly inclined, showing 

 the side of the spicule, indicated by shading. 



All figures drawn by Camera lucida, figs. 1 to 12 under ; Zeiss D, Oc. No. 4 ; figs. 

 13 and 14, Zeiss D, Oc. No. 2; fig. 15, Zeiss E, Oc. No. 3. 



