BASAL PLATES IN CRINOIDEA CAMERATA 505 



From a study of Figs. 1 and 9 it will be seen that eight types of 

 basal modification, by the reduction in number of the basal plates, 

 occur in the Camerata during the Paleozoic era. The primitive 

 base (a — b — c — d — e — ), Fig. 1, No. 1, appears in the Ordovician, 

 but it probably originated long before that time. The four-basal 

 type (a — b — cd — e — ), Fig. 1, No. 2, appears in the Ordovician, 

 becomes abundant in the Silurian, and disappears before the close 

 of the Devonian. The three- inequi-basal type (ab — c — de — ), 

 Fig. 1, No. 3, makes its appearance in the Ordovician, increases in 

 the Silurian, reaches its climax in the Devonian, and disappears 

 in the Mississippian. The three- inequi-basal type (ea — be — d — ), 

 Fig. 1, No. 6, is present during the Silurian, Devonian, and Missis- 

 sippian, but never becomes very prominent. The one-basal type 

 (abede), Fig. 1, No. 5, occurs at various times during the Paleozoic 

 period, but not as a generic or specific characteristic. The hexag- 

 onal, five-basal type (a — b — c — d — e — ), Fig. 9, No. 6, is present 

 in the Ordovician. The hexagonal, four-basal type (a — b — cd — e — ), 

 Fig. 1, No. 8, appears first in the Silurian (Richmond) and dis- 

 appears during the Silurian. The three- equi-basal type (ab — ex — 

 de — Y, Fig. 1, No. 10, appears in the Silurian, increases in the 

 Devonian, reaches its climax in the lower part of the Mississippian, 

 and then disappears. The two- equi-basal type (abc — xde — ),* 

 Fig. 1, No. 12, is introduced in the Kinderhook and becomes extinct 

 in the lower part of the Pennsylvanian. 



Possible combinations of the primitive five-basals: Since 

 Wachsmuth and Springer have assumed that all crinoids having 

 an unequally tripartite base have the smaller basal in the left- 

 anterior interray, it may be well to consider what combinations 

 may be expected from anchylosis of the primitive five-basals. 

 These combinations are explained below and illustrated in Fig. 5. 



Fig 5, No. 1, illustrates the primitive basal cup of pentagonal out- 

 line, represented by the formula a — b — c — d — e — , and found in 

 Glyptocrinus, Schizocrinus, Stelidiocrinus , and young individuals 

 of Platycrinus. 



Fig. 5, No. 2, shows a common Ordovician and Silurian type of 

 reduction (a — b — cd — e — ), in which the two anterior basals are 



1 Formula based upon Wachsmuth and Springer's theory. 



