542 HERRICK E. WILSON 



The first change of interest in anchylosis of basals is the change 

 of plate outline, from the pentagons through a heptagon to a hexa- 

 gon. With lateral anchylosis of two basal plates, the pentagons of 

 the basals are merged into a heptagon, with a re-entrant angle 

 where the supported radial interlocks with the supporting compound 

 plate. Upon further development, absorption decreases the angle 

 of the radial plate, causing it to assume, first a lower angle, then a 

 proximally convex outline, and finally, by complete absorption, a 

 straight angle. At the same time, by increased deposition along 

 the adjacent basal margin, the re-entrant angle of the heptagon is 

 gradually filled, the filling comforming throughout with the reduc- 

 tion of the inserted angle. The final step is the change of the 

 heptagon to a hexagon. 



At some time either before, during, or after anchylosis, a 

 remarkably persistent reduction of the compound plate or its 

 component parts occurs! This reduction is parallel to the line of 

 sutural closure, and is sometimes accompanied by reduction in the 

 proximal diameter of the directly supported radial. From the 

 principles set forth in the discussion of reduction and compensating 

 enlargement, either compensating enlargement of the adjacent 

 plates must take place or distortion follow the reduction. When 

 only one pair of basals is anchylosed, as in the Xenocrinus, etc., 

 and the Calyptocrinidae, the reduction is bilaterally symmetrical; 

 the basals adjacent to the reduced plates are equally enlarged, and 

 the reduction is apparently not due to deep-seated causes and 

 affects only the basal plates and the proximal margin of the apposed 

 radials (PL II, Nos. 2,7). When two pairs of plates are anchylosed, 

 as in the Stephanocrinidae, Pentremitidae, and Platycrinidae, the 

 problem is not so simple, for on one side the compound basals are 

 mutually apposed, and the intervening suture meets the center of 

 the proximal margin of the radial. If the reduction of the com- 

 pound basals is asymmetrical and occurs only on the sides opposed 

 to the simple basal, no distortion in symmetry is necessitated. If, 

 however, the reduction is symmetrical, there must be a distortion 

 in symmetry of the cup, for there is no basal to enlarge where the 

 compound plates are mutually opposed. In the Platycrinidae the 

 reduction is asymmetrical, and is perhaps due to superficial atrophy. 



