THE SHELL STEUCTURE 39 



the arenaceous and the cliitinous ; but these occup}^ 

 a subsidiary place in the chissification, for the genera 

 Avhich they comprise have representatives also in the 

 hyaline and even the porcellanous groups. 



1. Imperfoirifa or Porcellanea. — This type of 

 shell is, as the first name implies, without perfora- 

 tions in the shell wall, and the emission of the sar- 

 code filaments (pseudopodia) is effected by the oral 

 aperture only. The appearance of the shell is of a 

 chalky white, generally glazed, and somewhat like 

 porcelain. The shells, when seen by transmitted 

 light, either in thin sections or, in the case of thin- 

 shelled or young forms, w^hen mounted in a medium 

 like Canada balsam, appear of a pale horn-brown 

 colour. 



The mineral constituent of this type has long 

 been supposed to be carbonate of lime in the form 

 of aragonite. This appears from recent researches, 

 and from the author's own experiments, to be 

 extremely doubtful, and is more likely to be an 

 intermediate mineral condition in which the organic 

 element is intimately mixed with the mineral, and 

 probably corresponding with the new mineral species 

 Conchite. One powerful argument against the view 

 that the porcellanous shell is composed of aragonite, 

 which is a very unstable mineral, is the fact that 

 certain calcareo-argillaceous rocks of Carbo-Permian 

 age from Australia have recently been described, 

 which are largely made up of the tests of a wild- 

 growing or meandering form of Nuhecidaria in 

 which the shell texture is exactly comparable with 



