534 HERRICK E. WILSON 



gives to the plates their polygonal outline and must not be consid- 

 ered a form of atrophy. Plate contact does, however, produce 

 atrophy when the accelerated growth of one plate causes the 

 reduction in size of some adjacent plate. This type of inhibition 

 may be termed superficial atrophy, and is apparently the type 

 just illustrated in the absorption of the first costal in Alloprosallo- 

 crinus. Atrophy of the other type is apparently the result of 

 marked internal changes which appear on the exterior in the reduc- 

 tion of skeletal parts. This form of inhibition may be termed 

 deep-seated atrophy, and is the type illustrated in the drawing 

 together of the posterior radials in Pterotocrinus upon the reduction 

 of the anal plate (PL III, No. n.) 



Fig. 6. — Figures showing the reduction of the first costal in Alloprosallocrinus 

 conicus: i and 2, from specimen No. 9350; 2 and 3, from specimen No. 9357, in the 

 University of Chicago collection. 



With the decrease in diameter of a plate in a closed cycle there 

 must be (1) a compensating increase in the diameter of some plate 

 or plates in the same cycle, or (2) a decrease in diameter of some 

 plate or plates of the apposed cycle; otherwise the symmetry 

 of the cup will be distorted. The first principle is clearly demon- 

 strated by the increase in diameter of the first interbrachial plates 

 in Amphoracrinus 1 upon the gradual reduction of the proximal 

 portion of the second anal plate. The second principle is clearly 

 demonstrated in the reduction of the apposed compound basal and 

 radial of Zophocrinus. 2 



This form of change might be confused with vertical plate- 

 splitting followed by anchylosis of the parts to the adjacent plates 

 if the change were a sudden mutation and no knowledge of the 

 ontogenetic development obtainable; otherwise the phylogenetic 

 succession would show the factors involved. If the reduction of a 



1 Ref. 32, p. 197. 2 Ref. 6, p. 151. 



