426 Universitij of California Publications in Zoology [Vol. 18 



represented portion of this genus, both in species and in individuals, 

 in which there are fourteen epithecal plates, variations in the pat- 

 tern of articulations occur at only four points, and especially that, 

 with the exception of one possible combination of the plates of the 

 ventral region which is as yet unknown, cases have been found illus- 

 trating all of the combinations of the plates at each of these four 

 regions which are geometrically possible. 



Among specimens examined or among species already figured in 

 literature, examples have been found of all of the possible combina- 

 tions between figs. 1, 3, and 4 for the ventral patterns and figs. 5 and 

 6 for the dorsal patterns. The series of known combinations of the 

 dorsal patterns given in figs. 7 and 8 with the three ventral patterns 

 is incomplete, and from evidence which will be introduced later it 

 seems that the patterns of figs. 7 and 8 maj^ represent aberrant forms 

 or forms in whicli there may have been special attempts at adjust- 

 ment with unusual conditions of the environment. 



Plate Patterns, a Basis for Subdivision of the Genus, 

 Peridmium 



The fundamental importance of the skeleton in portraying phylo- 

 genetic relationships has already been discussed. On this basis we 

 venture to propose that the system of patterns outlined above presents 

 a means for not onl}- accurately describing the species of this genus 

 but also for relating them more or less closely according to a natural 

 order. 



Superficially it might be possible to divide the genus into groups 

 either according to the three known patterns for the ventral plates or 

 into four groups according to the dorsal patterns represented, and to 

 subdivide these primary divisions according to the several combina- 

 tions found between these two sets of patterns. There are certain con- 

 siderations, however, which make it seem reasonable to regard the 

 variations of the ventral plates as of more fundamental significance 

 than the variations of the dorsal plates. 



In genera apparentl}^ related rather closely to Peridimnm there 

 are two ways in which the number of plates is increased. These 

 genera all possess four whorls of plates — apical, precingular, post- 

 cingular, and antapical. The simplest method for increasing the 

 number of plates — and this increase seems to be the trend of special- 



