BASAL PLATES IN CRINOIDEA CAMERATA 541 



and Fistulata upon interpolation of the anal plate, and broadening 

 of the radials without trunkation upon interpolation of interbra- 

 chials, as in many of the Camerata; (3) increase in the body wall, 

 as is shown in the lengthening of the calyx of Acrocrinus upon 

 interpolation of the extra plates between the basals and radials; 

 (4) interpolation of an extra plate in an adjacent cycle or group 

 to satisfy the demands of plate alternation; (5) deformation 

 of the cup; (6) various combinations of the first four effects 

 named. 



/) Anchylosis. — The process of anchylosis is provisionally placed 

 under this group of processes, because of its intimate association 

 with certain modifications of plate contact and position which cast 

 some doubt upon the propriety of assuming it to be a result and 

 not the cause of those modifications. This fact is shown in the 

 following section. 



anchylosis: its antecedents and consequences 



I. ANCHYLOSIS AND REDUCTION 



Anchylosis is the most potent factor operative in the obliteration 

 of sutures, and while it has been discussed as a simple ontogenetic 

 process, its antecedents and results have not been considered. 

 The expression "reduction and anchylosis," so commonly used in 

 the description of brachials, means anchylosis following and depend- 

 ing upon reduction, but whether or not this usage is morphologically 

 correct is not clear to the writer. Anchylosis may take place 

 without reduction, or reduction without anchylosis, although the 

 former is not common. Anchylosis may perhaps be either preceded 

 or followed by reduction, but the writer is inclined to believe that 

 when anchylosis is preceded by reduction the reduction is phylo- 

 genetic, and that in ontogenetic development anchylosis is followed 

 by reduction. That is, plates which will appear in the adult as a 

 reduced anchylosed unit are in ontogenetic development up to the 

 time of anchylosis the equivalent of the other plates in the same 

 cycle or series, and with anchylosis inhibition of growth causes the 

 reduction of the compound plate. This inhibition of growth may 

 be due either to deep-seated atrophy in local areas or to local 

 superficial atrophy. 



