1 6 FREDERICK IV. SARDESON 



may in some species support two or more acrogene growths, and 

 in just these cases also the acrogene part may be small or want- 

 ing, arguing that the basal expansion might have been the origin 

 of some strictly perigene species, the acrogene part being wholly 

 suppressed. The laminate form may have again become massive. 

 Finally, an acrogene growth may be round, compressed, flattened, 

 frond-shaped, or bifoliate. In short, the series between zoarial 

 forms is very complete, and genetic relationship between the 

 most extreme forms may be presumed. The lines of evolution- 

 ary development have never been traced, however, and they 

 evidently cross or parallel in a confusing manner ; hence this 

 character is of taxonomic use in species chiefly. 



The different zoarial forms result not from changed shape and 

 size of the component cells, as one can readily observe in similar 

 zoaria of extremely different cells, but, as seen firstly in varia- 

 tions of a species, it results from increase of cells, the region of 

 greatest cell increase being that of greatest zoarial growth, and 

 inversely. The change of zoarial form, nevertheless, must be 

 made to explain the change of cell as seen in different regions 

 in the same species. Thus ; in the acrogene growth, where the 

 cells are turned from the axis of rapid growth to the peripheral 

 slow growth region, it changes markedly, becoming thick walled, 

 closer tabulated, etc. (PI. A, Fig. 6, c). Noticing that the cell 

 apertures do not spring apart under any circumstances, and 

 presuming that this is because the respective zooids were bound 

 together by a cortex, it can be understood how the same cell 

 can be different in two parts, and why there are certain differ- 

 ences in cells. Thus, the tip of an acrogene zoarium, like a 

 hemispheric or massive zoarium, has the cells subparallel, length- 

 ening as new cells develop, so that the surface or circumference 

 widens and the radius or cell-length increases proportionately. 

 But as a cell turns into the peripheral region it comes into a 

 zone of more restricted circumference and radial lengthening; 

 hence the cell is shorter, thicker-walled, and closer-tabulated. 

 Moreover, the cell-increase lessens to some degree, which further 

 restricts the circumference and radius with added effect. In the 



