part 8] AXD STRUCTURE OF THE FORAMINIFERAL SHELL. 197 



conchite, with a specific gravity of 1*4 we may calculate the specific 

 gravitj^ of the purely mineral ingredient. It comes to 2'742. 

 But the published analysis of Orhitolites shows that 98*27 per cent, 

 of this ingredient consists of 9 "55 per cent, of magnesium carbonate 

 and 88*72 per cent, of calcium carbonate, and thus its specific 

 gravity should be 2*742. That this number should be identical 

 with the preceding is an accident, since no account has been taken 

 of the traces of alumina and iron peroxide present in the shells ; 

 but the agreement is sufficiently in harmony with Dr. Lister's 

 conclusion founded on a chemical method. 



A more fundamental difference between the Perforate and the 

 Imperforate shell is provided by the minute structure of its wall. 



It is well known that the wall in simple forms of the Perforata 

 is found, when examined between crossed nicols under the micro- 

 scope, to consist of rods of calcite arranged with their principal 

 axes directed parallel to the pore-canals : that is, with their optic 

 axis normal to the surface, so that a spherical chamber, such as 

 occurs in Orhulina or Glohigerina, may be regarded as built up of 

 prisms, each with its optic axis corresponding to a radius of the 

 sphere. Hence, between crossed nicols, it presents a dark, well- 

 defined cross which remains stationarv on rotation of the stage 

 of the microscope.^ A petrologist might regard it as a hollow 

 spherulite. That the optic axes lie normal to the surface is shown 

 by the optical sign which, as S. Awerinzew ^ was the first to show, 

 is negative. 



In the most complicated forms of the Perforata, such as Num- 

 mulites, the fundamental skeleton is constituted according to the 

 same law, and this is true also of Calcarina and Tinoporus, but I 

 am unable to speak in detail of the supplemental skeleton, which 

 requires further examination. 



The Perforate structure generally survives the changes which 

 accompany fossilization, and it frequently but not always de- 

 termines the crystallographic orientation of the calcite which is 

 deposited within and around the test after death.^ Simple forms 

 then present the dark cross as plainly after fossilization as before. 



^ It may be observed in passing that coccolitlis (Discoliths and Cyatho- 

 liths) when examined in this way also give a dark cross ; the arms of the 

 cross are, however, not always straight, but frequently curved in a manner 

 suggestive of a slightly spiral arrangement. The illuminated segments 

 between the arms contrast by their brilliance with the dark field of the micro- 

 scope. Advantage may be taken of this fact when one is searching for 

 coccoliths dispersed through fine sediment. 



' ' Ueber die Struktur der Kalkschalen Mariner Ehizopoden ' Zeitschr. f . 

 Wissensch. Zool. vol. Ixxiv (1903) p. 478. 



'^ It is not only among the Perforate Foraminifera that the molecular 

 structure of the skeleton persists throughout fossilization, and dominates that 

 of any subsequently-deposited calcite. The spicules of a calcareous sponge 

 (which are composed of calcite), when placed in a solution of dihydric calcium 

 carbonate, become covered with a growth of calcite which crystallographically 

 is merely an extension of the original spicule. The ossicles of Echinoids 

 furnish a more familiar example. 



