384 



Scientific Proceedings, Royal Dublin Society. 



of polarised light ; but this introduces questions too wide for profit- 

 able discussion in this place. 



a 



Fig. 4. Fig. 6. 



Showing the positions in which the spicules of Grantia conipressa extinguish between crossed 

 nicols. o a, line of intersection of the plane of the principal section of the analyzer with 

 the spicule: the optic axis lies in this plane. 



Finding in my collection a slide on which I had mounted spi- 

 cules from an unidentified calcisponge from Port Elizabeth/ possess- 

 ing acerates of a colossal size, I turned to examine the extinction 

 angles for these, and obtained results closely agreeing with those 

 for Grrantia. The sagittal spicules of this sponge are distinguished 

 by a particularly long unpaired ray : out of 10 examples, 5 ex- 

 tinguished at 0°, and 5 at 10° ; the acerates extinguished at from 

 13° to 19°. 



Etch-figures. — As these spicules had lain for some years in bal- 

 sam, which is known to dissolve calcite, I expected to find etch-figures 

 which, from the slow rate at which they would have been produced, 

 should be neatly .developed. Under a Zeiss D, and Ocular No. 4, 

 they were easily found on both acerate and sagittal spicules. On 

 the former they exist as parallel strise, inclined at an apparently 

 somewhat variable angle, often of 60° to 75°. The strise frequently 

 occur in groups, which terminate in many cases in a ledge trans- 

 verse to them, making with them sometimes an apparent angle of 



^ In all probability this sponge is Lenconia glomerosa (Bwk.), P. Z. S., 1873, pt. iv., 

 figs. 1 to 6, 



