SoLLAS — On Sponge Spicules. 391 



than that of ealeite. Either it consists of aragonite, or, if of 

 calcite, some other substance must be present in addition, possibly 

 iron carbonate or phosphate of lime. It would require 12*5 per 

 cent, of organic matter to reduce the specific gravity of this forami- 

 nifera from 2"9 (that of aragonite) to 2-725, and 14-37 per cent, to 

 2-7, the specific gravity which most of them possess. 



Although sufficiently common in the tertiary rocks, Imper- 

 forate foraminifera are much rarer than the Perforate in older 

 deposits, and this comparative rarity would be consistent with 

 an aragonite constitution for their tests. 



Dr. Carpenter^ states that the shells of the Imperforata have 

 no structure of any kind. I find, however, that the shell of 

 Milliola, when viewed mounted in turpentine with transmitted 

 light, has a very finely granular structure, minute granules lying 

 thickly and evenly scattered in the transparent amber matrix. 



NOTE ADDED IN THE PEESS. 



In order to make quite sure that the outline of the transverse 

 section of the acerate spicules in some species of calcareous sponges 

 is otherwise than circular, I prepared thin slices of entire specimens of a 

 species of Grantia from Vancouver's Land {Grantia coronata?) by cutting 

 in frozen jelly. The illustration (pi. xv., fig. 13) shows a section 

 through the "neck" of the sponge ; the coronal spicules are arranged 

 in concentric circular rows (arcs of nine rows out of sixteen visible are 

 represented in the figure), and the transverse section of each is an 

 ellipse with flattened sides, or an oblong with rounded ends. Certain 

 larger acerates of the radial tubes (pi. xv., fig. 14) offer polygonal out- 

 lines, like those of the Pharetrones (pi. xv., fig. 12). These sections ob- 

 tained from spicules, unaltered in any way by treatment, dispose of all 

 doubt as to their form ; and since the acerate spicules of three species of 

 CalcispongiEe, all that I have examined, give similar results, it would 

 appear that a circular outline is much less common among the acerates 

 of this group than has been hitherto supposed. It would appear that 

 the form is influenced by the mineral composition, and if so, one might 

 expect calcareous spicules to rarely exhibit quite circular outlines : 

 other duties, however, prevent my extending my observations in this 

 direction. 



^ Carpenter, Introduction to the Study of the Foraminifera, p. 44. 



