496 HERRICK E. WILSON 



the evolution of the basal plates in the monocyclic Crinoidea, 

 and the result of this investigation will be stated in the latter part 

 of this study. 



REVIEW OF CERTAIN PHYLOGENETIC CHARACTERISTICS IN 

 MONOCYCLIC CAMERATA 



In this review the phylogenetic characteristics of the various 

 groups of monocyclic Camerata will be considered, for it is in this 

 order that the succession is better understood and that the evolution 

 of the basal plates is the most complex. There exist in this order 

 two great groups, separated structurally but not phylogenetically 

 according to the outline of the basal cup, and therefore according 

 to the presence or absence of an anal plate in contact with and 

 trunkating the posterior basal. The first of these groups possesses 

 an anal plate in apposition with the posterior basal, and has in 

 consequence a hexagonal basal. The second group, in which plates 

 of the anal series are either present or absent but not in contact 

 with the posterior basal, has a pentagonal base. In considering 

 these facts, questions of descent and relationship necessarily arise. 



In the ontogenetic development of the external skeleton in the 

 living Antedori 1 so many of the phylogenetic steps found in fossil 

 crinoids are illustrated, that it may well be looked upon as a key 

 to their methods of development and perhaps to their interrelation- 

 ships. Leaving out of consideration the earlier embryo-logical 

 stages, let us follow in detail ,the methods of plate intercalation 

 and development after the formation and contact of the basal and 

 oral plates (Fig. 2). 



About the time of the attachment of the pentacrinoid larva, 

 the basal plates assume a regular, trapezoidal outline, the lower 

 part of each being an acute-angled triangle with its apex distally 

 directed (Fig. 2, No. 1). The sides of the lower triangle are bor- 

 dered by a somewhat thickened edge of solid, transparent stereom, 

 the presence of which indicates that the plate has received its full 

 proportionate increase in that direction. 2 Even after the plate 



1 Antedon, while more highly specialized in its larval development than some of 

 the other modern crinoids. is chosen for comparison with the Camerata because it is 

 the best known of any of the genera in which the larval development has been studied. 



- Ref. 11, p. ~2Q. 



