SoLLAs — On Sponge Spicules. 377 



r^ will never, however, be tMs minimum ; but for light passing 

 perpendicularly through a cleavage fragment lying with one face 

 on the microscope slide is about 1'56. If now both minerals be 

 viewed mounted in Canada balsam [r. i = 1-525), or oil of cloves 

 (r. i = 1*535), most of the calcite will never cease to show relief 

 as it is rotated between the nicols ; the aragonite, however, under- 

 goes a marked change; parallel to the principal section of one 

 nicol it stands out in bold relief, like a projecting rock; as it is 

 rotated, it begins to flatten down, the brilliancy about its edges 

 and angles fades, and, finally, when it has been rotated 90°, it has 

 lost all relief, and appears as the merest film flattened out in the 

 plane of the glass slide. The experiment is a striking one, and 

 the discrimination between the two minerals may in this way be 

 made in a very short time — a few minutes will sufiice. 



Another application will be found later on, where the refrac- 

 tive index of a calcite spicule lying in a definite position, having 

 been determined for the extraordinary rays, the position of the 

 optic axis can be deduced. 



Specific Gravity. — By a method described later on, the specific 

 gravity of the acerate spicules of a Eenierid sponge, of a frag- 

 ment of a Lithistid sponge skeleton, and of various diatoms, was 

 found to be 2*04, a result in close agreement with that found 

 by M. Thoulet for Hexactinellid sponge spicules. The reference- 

 minerals (a term which will be understood after reading the 

 description of the method) employed in this case were crystals 

 of sulphur deposited from solution in carbon disulphide, and 

 fused sulphur: the specific gravity of the latter is 1*98; of the 

 former, 2*05. 



Calcareous Skeletons. — Amongst the fossil sponges are some 

 which, in 1877, offered great difficulties to the palaeontologist. 

 Professor Zittel regarded them as calcisponges, while I was 

 inclined to group them with the siliceous sponges, at the same 

 time expressing great uncertainty on the point. ^ Since then, the 

 discoveries of Hinde and Dunikowski have thrown more light on 

 these previously-obscure " Pharetrones," and I have now not the 

 least doubt that they must be relegated to the Calcispongise. 



1 Sollas, "On the Genus Catagma," Annals and Mag. Nat. Hist., ser. 5., vol. ii., 

 p. 363. 



SCIEN. PROC. R.D.S. VOL. IV. PT. VII. 2K 



