378 Scientific Proceedings, Royal Dublin Society. 



Pharetrospongia, however, is not one of the Pharetrones, and is 

 as certainly a siliceous sponge as they are calcareous. 



Some observers have gone so far as to deny the possibility of a 

 calcisponge being preserved in the fossil state, alleging that cal- 

 careous spicules, when kept in distilled water, decompose in the 

 course of a year ; as, however, I have calcareous spicules in my 

 possession, which are as perfect now as they were originally, when 

 placed in distilled water ten years ago, the force of this argument 

 is weakened. 



It becomes interesting to inquire, however, into the mineral 

 state of calcareous spicules, i. e. whether they are composed of 

 calcite, or aragonite, or of some combination of calcareous and 

 organic matter. If we find that they consist of calcite, we shall 

 have no difiiculty in concluding that they may well be preserved 

 in a fossil state, but the contrary, if they consist of aragonite ; or, 

 at all events, in the latter case their preservation will be rare and 

 exceptional, and their structure will wholly disappear. As every- 

 one, without inquiring into the matter, seems to have regarded 

 them as calcite, I was quite prepared to find the contrary ; the 

 result, however, of my observations will appear in the sequel. 



Refractive Index. — Most of the acerate spicules of Qrantia 

 ciliatia and Grantia compressa, the species with which I chiefly 

 worked, disappear when observed between crossed nicols, in linseed 

 oil, along one line of extinction (light being restored by a quartz- 

 plate) : along the other line of extinction, at right angles to the 

 former, all the spicules disappear when viewed in carbon disulphide 

 containing a little sulphur; their refractive indexes are then t = 1'485 

 and u) = 1'650, most nearly agreeing with those of calcite. 



The invisibility of the acerate spicules when mounted in linseed 

 oil, and at the proper angle of extinction, is often absolute ; they 

 cannot be seen even with strong illumination under a Zeiss 0. 



Specific Gravity. — The difiiculty of determining the specific 

 gravity of calcareous sponge spicules by the methods of weighing, 

 I found insuperable. They are so small, and so difficult to free 

 completely from air, even with an air-pump, that I had to adopt 

 some other method. Sonstadt's solution appeared to offer the 

 best chance of success ; but here again the small size of the spicules 

 was a difficulty. This, however, was overcome by adapting the 

 Sonstadt method to use with the microscope. An ordinary col- 



