BASAL PLATES IN CRINOIDEA CAMERATA 491 



processes operative in the growth and modification of many groups 

 of invertebrates. 



To Professor Weller, Mr. Springer, and Mr. Clark the writer 

 is especially indebted for their many kindnesses, their helpful 

 advice, and the privilege of freely studying their collections of 

 fossil and recent crinoids. 



REVIEW OF WACHSMUTH AND SPRINGER' S THEORY OF 

 BASAL PLATE EVOLUTION 



While various phases of basal plate evolution have been touched 

 upon by numerous writers, the discussion by Wachsmuth and 

 Springer 1 is the only one in which a general treatment of the sub- 

 ject has been undertaken, and, in order that the reader may have 

 their theory clearly in mind before going farther, it will be quoted 

 in substance. 



The base of a monocyclic crinoid is composed of a single cycle 

 of plates, termed the basal plates, lying at the proximal end of the 

 cup, between the stem and the first plates of the radial series. 

 This plate cycle was primarily composed of five separate plates, 

 but, by anchylosis or the union of two or more members, they were 

 reduced from five plates to four, three, two, or one. The first 

 monocyclic crinoids had five basals. 



Before the close of the Lower Silurian (Ordovician) there appeared two 

 monocyclic genera with four basals, both having a special anal plate inter- 

 posed between the radial. The quadripartite base reached its culmination 

 in the Upper Silurian (Silurian), and disappeared before the close of the 

 Devonian. The earliest genera with a tripartite base occur in the Upper 

 Silurian; some of them have an anal plate, and others not. When that plate 

 is represented, the basals are of equal size; when absent, two of the basals 

 are equal, and the third about half smaller. The two forms continue to 

 exist side by side to the end of the St. Louis group of the Carboniferous (Missis- 

 sippian) when both became extinct. The bipartite base is restricted to the 

 Carboniferous (Mississippian and Pennsylvanian) . It occurs from the Kinder- 

 hook group up to the Coal Measures, but is found only among genera with a 

 large 2 anal plate. 



1 Ref. 39, pp. 52-68. 



2 The exact meaning of the statement that the bipartite base is found only among 

 genera with a large anal plate is doubtful, as Pterotocrinus, a genus with a hexagonal 

 bipartite base, usually has, in proportion to the size of its basals and radials, the 



