DULLIN MICROSCOPICAL CLUB. 303 



tensely opalescent, of a darker colour, and distinctly banded 

 (bands of deposition), numerous short, irregular fissures or scars, 

 generally curved and intersecting, were observed. No light, how- 

 ever, was thrown upon the origin of the opalescent property of 

 the mineral until the slice was examined with the aid of the 

 polariscope, the Nicol's prisms being crossed, and with a very 

 strong light. Under these conditions the structure of the mineral 

 became splendidly evident. The whole surface was seen to be 

 diversified by short, wedge-shaped bands of green, crimson, pink, 

 yellow, and gold colours, rudely parallel, pointing inwards from 

 the edge of the specimen, and transverse to the " bands of deposi- 

 tion" above described. In favorable positions distinct fissures, 

 inferred to be due to the intersection of oblique planes with the 

 plane of the thin slice, were observed to coincide with the changes 

 of colour, and it became evident that the opalescence was 

 intimately connected with, if not directly due to, the existence of 

 such planes. 



Mr. Hull considered these planes to be due to shrinkage, 

 originated upon the formation of mineral by aqueous deposition 

 in the cells of the tufa. In some cases the fissures appeared to 

 be wedge-shaped, or to converge from opposite directions upon 

 other predominant lines of fissure, so that the corresponding planes 

 of shrinkage would form minute, irregular prisms, giving rise to 

 the play of colours as the light was reflected from the planes and 

 refracted through the mineral in various directions. 



These observations appeared to bear out, to some extent, the 

 views of Sir David Brewster, who attributed the calorific eflfects 

 of opal to "minute pores or vacuities arranged in parallel lines 

 forming planes, various such planes being placed close to each 

 other, so as to occupy a space with three dimensions." He also 

 adds, " these pores exhibit a crystalline arrangement like the lines 

 of sapphire" (' Edin. New. Phil. Journ.,' xxxviii, 1815). 



Seed of Castilleja indivisa, exhibited. — Mr. Pirn showed the 

 seeds of Castilleja indivisa. The seed was invested with a large 

 and beautifully reticulated testa similar to that of Pyrola, 

 forming a netted sac many times louger than the contained 

 embryo. 



Structure of Spines of Cidaris tribuloides, Lamk., O. metularia, 

 Blainv. and C. Thouarsii, Val. — Mr. Mackintosh exhibited trans- 

 verse sections of the spines of the above three species, the only 

 ones retained in the genus by A. Agassiz in his recent ' Revision 

 of the Echini.' The first of these can readily be discriminated 

 by the open nature of the external crust, in this resembling the 

 spines of Goniocidaris geranoides, Lamk. (Club. Minutes, August, 

 1876). The other two have a more solid crust, but in C metularia 

 the axial reticulation has thick bars, whilst in C. Thouarsii they are 

 very slight. 



Abnormal leaves of a species {or var.) of Abies from Japan, a 

 po.'isible hybrid between A. bifida and A. firma. — Dr. McNab 

 exhibited sections of the leaves of a pine from Japan. The speci- 



