PROBLEM OF THE MONTICULIPOROIDEA I 55 



intermediate to P. foliata and Rhinidictya, but one of this genus 

 may suffice. 



Rhinidictya mutabilis Ulr., is bifoliate, with autocells o.2 mm 

 or less in diameter (Plate B, Figs. 4, 5, and 6). It begins 

 apparently as a small thin vertical blade I or 2 mm wide, with a 

 small -basal expansion. It grows in height rapidly, increasing 

 more or less in apical width and branching to the length of 200 mm 

 or less. As the zoarium enlarges, its basal expansion widens 

 slowly, the cell apertures filling solid. The base or oldest part of 

 the stalk becomes thick, more or less cylindrical, and solid sur- 

 faced. The zoarial branches thicken slowly, having ceased cell 

 increase and built solid narrow margins, the ends either mean- 

 while growing by cell increase or having finally ceased likewise. 

 Furthur development consists only in thickening of zoarial 

 parts. 



Cell increase and greatest zoarial growth takes place only at 

 the apices of the frond and margin of the basal expansion, as in 

 Pacliydictya. The width of any branch or part depends mainly 

 on the relative growth vigor at the apex in the building of that 

 part. The thickness depends a little on vigor of axial growth 

 but mainly upon age and cell lengthening. At the same time 

 the solid margin extends, so that some width is added to that of 

 the first growth. The initial parts appear to have been small, 

 narrow, dichotomously branching. Some individuals dwarfed 

 or matured at this stage. Others grew long branches, 2 to 8 mm 

 wide, dichotomous or palmate digitate. Just below the forks 

 the zoarium is widest, and there a macula appears on medium 

 width branches or a row of maculae continue down the middle 

 on wider ones, or two to four rows on palmate parts. 



The cells alternate in vertical rows. They are somewhat 

 smaller in the initial than in the later zoarial growth stages, but 

 as a rule the widths of the zoarium correspond to numbers of 

 cells and cell rows. Increased number of rows is less by inter- 

 calation than by marginal addition at the side of the growing 

 apex, and decrease is by reduction at the margin. Sometimes 



