"The natural resistance of many minute structures in samples of various sediments is very great. 'Microforaminifera' are recognizable even after natural decalcification, when they possess membranous linings or when their chamber walls have been replaced with resistant mineral matter at an early stage of sedimentation. In addition, the microfossils commonly associated with them are not 'polospores'. . . , but various flagellate Protista, preserved because of the characteristic organic material of which their tests are composed."