552 HERRICK E. WILSON 



not originate in the position in which they now originate, and too 

 much dependence should not be placed upon this character. The 

 evidence presented by fossils is fixed, although often wrongly inter- 

 preted, and until stronger evidence is submitted it is well to hold 

 closely to that presented in the stratigraphic succession. 



Stratigraphically the pentamerous base preceded the hexamerous 

 base, and when we consider with this the evolution of the two- 

 basaled, hexagonal Camerata, a more logical theory of development 

 for the anal plate is presented. In this short-lived group we have a 

 very rapid evolution from some platycrinid stock. The anal plate 

 in Platycrinus originates in the posterior interray between the distal 

 margins of the radial plates, while the anal plate in Dichocrinus 

 projects above the level of the radials and costals and bends sharply 

 inward toward the anal tube; its distal portion is reduced, and, if 

 it were separated from the enlarged proximal portion and slightly 

 modified, could not be distinguished from the anal in Platycrinus. 

 Enlargement and downward growth of the anal seems then to have 

 occurred, and a different theory is offered for the interpolation of 

 the anal plate. 



This theory for the appearance of the anal plate is, then, that 

 the anal plate is of secondary derivation; that it was interpolated 

 phylogenetically after closure of the radial cycle, but ontogenetically 

 after the basal had formed a closed cycle and before the radials 

 had come into contact. The cup in this stage of development is 

 in a flexible condition, and readjustments can readily be made. 

 Development of the anal plate in this position requires no true 

 migration to come into contact with the posterior basal; portional 

 migration, or proximal growth with distal inhibition in the younger 

 stages, will produce the Tanaocrinus type of anal, while distal 

 growth alone is necessary to produce the Glyptocrinus type of anal. 

 The stimulus which kept the radials apart and permitted this 

 downward growth of the anal plate was the demand for room on 

 the part of an enlarging hind-gut, and it is this stimulus which has 

 caused some of the most remarkable changes in crinoid evolution. 

 Additional plates in the anal series were probably added as needed, 

 for protection of the anal tube, and no slipping downward of a 

 completed series of anals is required. In considering this change 



