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IX.— NOTE ON THE ARTIFICIAL DEPOSITION OF CRYSTALS 

 OF CALCITE ON SPICULES OF A CALCT-SPONGE. By 

 PROFESSOR SOLLAS, D. Sc. 



[Eead, June 15, 1885.] 



Some acerate and triradiate spicules of a calci-sponge, after having 

 been left to stand for some days in water containing an excess of 

 calcium carbonate, were found to have become incrusted with an 

 abundant crop of minute crystals of calcite. The exact form of 

 the crystals was not ascertained; but, as on rotation between 

 crossed Nicols, they extinguished simultaneously with the spicules 

 on which they were seated, and underwent the same changes in 

 refractive index, we may conclude that the optic axes of the cal- 

 cite forming a spicule, and the crystals incrusting it, are similarly 

 orientated. 



A curious feature in the distribution of the crystals is worth 

 notice. They do not cover the whole of a sagittal triradiate, but 

 are confined to opposite sides of the paired rays and the extremity 

 of the unpaired ray ; an acerate, is, however, often covered with 

 them for its whole length, but usually only on opposite sides. 

 Thus the crystals are deposited only on those regions which show 

 the greatest liability to solution: 1 thus it would appear that the 

 polarity which leads to solution also determines deposition. 



1 Vide Sollas, on "Physical Characters of Calcareous Spicules," &c, Proceedings 

 Royal Dublin Society, vol. iv., N. S., p. 385. 



SCIEN. PKOC, K.D.S. — VOL. V. PT. II. 



