The Dichotomous Key...

Are the cells rectangular and transparent ( 1 ) or square, is the cell diameter between 1.0 and 1.2 micron and is there not indentations at the septa ( 2 ) or barrel, rectangular or discoid, is the cell diameter between 1.2 and 2.0 micron and is there indentations at the septa ( 3 ) ...?

Both the average diameter and its range in micron should be measured; it is important to note whether the diameter is greater than 1 micron or less than 1 micron.