112 THE FOEAMINIPEEA 



the misymmetrical forms of the Porcellanous and 

 HyaHne groups, as Nuhecularia and Bamiilina. In 

 external characters it also bears some resemblance to 

 the fossil genera HtacJieia and Nodosinella, but differs 

 in its simple chamber cavities and thinner shell- 

 walls. 



Halipliysema is another example of the types 

 which construct their tests largely of sponge spicules. 

 The bristling termination of the test causes the 

 organism to look very like a tiny sponge ; and it was, 

 indeed, taken for such by Bowerbank, who first 

 described it as the smallest known British sponge. 



Some doubt has been expressed regarding the 

 validity of some of the more irregular of the fore- 

 going examples of arenaceous organisms as definite 

 rhizopod structures, and Professor Haeckel is dis- 

 posed to regard them as members of the group of 

 deep-sea keratose sponges (Ammocoiiidce). In point 

 of size, for instance, these arenaceous forms greatly 

 differ from the Keratosa, being much smaller, even 

 when fully developed. 



The true forms of arenaceous Foraminifera are 

 practically unknown to us from the oldest rocks, and 

 there is, therefore, no direct evidence that these were 

 the primitive forms of foraminiferal life, although 

 strongly maintained by some writers. The oldest 

 Foraminifera known to us, as will be seen in the 

 chapter on the Geological Kange of these organisms, 

 are those of the hyaline group. This is to be 

 expected as a natural sequence in their relations to 

 the evolution of marine deposits and their accom- 



