EVOLUTION OF THE BASAL PLATES IN MONOCYCLIC 

 CRINOIDEA CAMERATA 



HERRICK E. WILSON 



United States National Museum, Washington, D.C. 



PART III 



EVOLUTION OF THE BASE IN MONOCYCLIC CAMERATA 



I. EVOLUTION OF THE PENTAGONAL BASES 



Having assumed that the anal plate plays no part in the evolu- 

 tion of the pentamerous base in the monocyclic Camerata, the 

 succession of changes modifying this base may now be considered. 



Starting with the simple pentagonal base a — b — c — d — e — 

 (Fig. 9, No. i), the first change noted is the reduction in number 

 of basals from five to four. It is evident that this change is due to 

 the anchylosis of but one pair of plates, either the anterior pair 

 or occasionally in Melocrinus the sinistro-laterals. Accompanying 

 this change there is usually a symmetrical reduction of the com- 

 pound plate and an asymmetrical reduction of the adjacent basals. 

 The formula for the base so developed is a — b — cd — e — (Fig. 9, 

 No. 2) for the anterior anchylosis and a — be — d — e — for anchy- 

 losis of the sinstro-laterals (Fig. 5, No. 2a). The next step is the 

 reduction to three unequal basals by the anchylosis of two pairs 

 of basals. In this combination the simple basal may be any one 

 of the five primary plates, although the left- or right-anterior plate 

 is usually the simple one. Anchylosis is here accompanied either 

 by asymmetrical reduction of the compound plates on the side 

 apposed to the simple basal or by deep-seated and symmetrical 

 reduction. In either case the enlargement of the simple basal is 

 usually symmetrical. Any of the three-basal forms might arise 

 from any of the possible combinations of four basals, provided 

 that the anchylosed pair in the four-basal form appears as one of 

 the compound units in the three-basal form. Thus the four-basal 

 form a — b — cd — e — might have given rise to a type in which either 



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