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THE AMERICAN NATURALIST [Vol. XLIX 



SUMMAKY 



Among the recent crinoids any wide departure from the 

 normal close approximation to true pentamerous sym- 

 metry indicates unfavorable conditions of one or other of 

 two main types, which are not mutually exclusive. 



These two types are 



1. Internal unfavorable conditions, induced by incip- 

 ient phylogenetieal degeneration through type-senescence, 

 as in the Plicatocrinidae, which in the recent seas repre- 

 sent the almost exclusively paleozoic Inadunata ; and 



2. External unfavorable conditions, taking the 

 form of 



(a) Phylo gc net) call y c.rccssive cold, which, to 

 cite one example, appears to be the determining 

 factor in the asymmetry of the genus Promacho- 

 crinus; or of 



(b) Phylo genetically excessive warmth, which 

 appears to be the determining factor in the asym- 

 metry of the family Comasterida?. 



