EVOLUTION OF THE BASAL PLATES IN MONOCYCLIC 

 CRINOIDEA CAMERATA. II 



HERRICK E. WILSON 

 United States National Museum, Washington, D.C. 



PART II 



2. CHANGES PRIMARILY MODIFYING THE RELATIONS OE PLATE 

 CONTACT AND POSITION 



The second series of changes, those which modify the primary 

 position and relation of the basals and radials, will now be con- 

 sidered. These are: (a) reduction and compensating growth; (b) 

 enlargement and compensating reduction; (c) plate division; 

 (d) plate migration; (e) plate interpolation; and (/) anchylosis. 



a) Reduction and compensating growth. — Reduction, or the 

 diminution in size of a plate, may be either a function of the absorp- 

 tive ameboid cells (see p. 502), or due to inhibited growth (atrophy). 

 That is, the absorption of a fully outlined plate may take place, 

 as in the absorption of the anal and oral plates in Antedon, or a 

 continuous diminution in development to a former standard of 

 size may result in the atrophy and final disappearance of a plate, 

 as in the great reduction of the basals in Pisocrinus quinquelobus 1 

 and the disappearance of the first costal in some specimens of 

 Eucalyptocrinus rosaceus 2 and Alio pros alio crinus conicus? Atrophy 

 in plate growth may be due either to plate contact, which inhibits 

 the free branching and anastomosing type of development, or to 

 some deep-seated morphological change. The simplest form of 

 inhibition in plate development is that shown in the normal growth 

 of plates after coming into mutual contact. It is the process which 



1 Ref. 5, p. 27. 



2 Ref. 28, p. 90, PL XI, Figs. 6, 7. 



3 In Alloprosallocrinus conicus the writer has found that the apparent anchylosis 

 of the costal plates (see ref. 39, p. 407) is due to the complete reduction of the first 

 costal. 



533 



