390 Scientific Proceedings, Royal Dublin Society. 



would govern the position of the crystalline axes of all the rest, 

 and as growth took place by apposition, all subsequently deposited 

 rhombohedra would be arranged in perfect conformity with those 

 which served as a foundation ; and should the spicules after the 

 death of the sponge serve as nuclei on which a further calcitic de- 

 posit was laid down, the crystals of this would be governed in their 

 arrangement by that already existing in the spicules/ Hence, in 

 fossil Pharetrones we should expect to find just the same optical 

 characters as those observed in the recent calcispongise. 



Foraminifera. — Dr. Sorby^ states that from the nature of the 

 case he was unable to determine the specific gravity of Foramini- 

 fera ; but with true insight he inferred that they consist of calcite 

 and not of aragonite. Dr. Sorby, no doubt, had in mind the 

 Perforate group of foraminifera; and I find by my method of using 

 Sonstadt's solution under the microscope, that these have a specific 

 gravity of from 2' 626 to 2'674. The forms I examined comprised 

 Globigerina, Textularia, Rotalia, Discorbina, Nodosaria, Nonionina, 

 and Lagena. They could be clearly seen under the microscope 

 floating free from air and other impurities in the Sonstadt solu- 

 tion, so that had it been desired I could have taken the specific 

 gravity of any single one. I also obtained the specific gravity of 

 Polytrema ; it is the same as that of quartz (2 •65). In the ma- 

 jority of the Perforate foraminifera the specific gravity is from 

 2"65 to 2*674 ; they evidently consist of calcite, and the difference 

 in specific gravity may be attributed to the presence of organic 

 matter, of which it would require 3 "3 per cent, at a specific gravity 

 of 1'5, to reduce the specific gravity of the foraminiferal test from 

 2'7 to 2'66. The Imperforate foraminifera are denser than the 

 Perforate forms. I examined Milliola, Peneroplis, Spirolina, and 

 Orbiculina. Their specific gravity ranged from 2*7 to 2*722 ; and 

 as they contain a large quantity of organic matter, easily seen as a 

 thin film when the mineral matter is dissolved away with acids, the 

 specific gravity of the mineral part of their tests must be higher 



^ This prediction has since heen unexpectedly verified. Some calcareous spicules, left 

 to stand in water containing carbonate of lime, have become overgrown with crystals of 

 calcite, arranged in accordance with the crystalline structure of the spicule, as ascertained 

 above. 



"^ Sorby, "Presidential Address on Limestone," Quart. Jour. Geol. Soc, 1879. 



