No. .585] 



A STUDY OF ASYMMETRY 



527 



given in the order of the extent of their departure from 

 the normal pentamerous symmetry: 



The asymmetry of the Comasteridae is considered more 

 fundamental than that of Promachocrinus for the reason 

 that it is characteristic of practically the entire family, 

 and also because it results in a much greater degree of 

 irregularity. It is interesting to note that asymmetry 

 of Type 3 is not uncommon among the Comasteridse, in 

 the form of individual variation. 



The asymmetry of the Apiocrinidae and Holopodidae is 

 considered more fundamental than that of the genus 

 Thaumatocrinus for the reason that it affects the entire 

 family, at the same time inducing a greater departure 

 from the normal form. 



The asymmetry of Rhizocrinus is considered less funda- 

 mental than that of Thaumatocrinus because, though 

 affecting all of the species, exactly as in Thaumatocrinus, 

 it is less extensively developed. 



The asymmetry of Monachocrinus affects only one of 

 the seven species of the genus. 



Briefly stated, it appears that, no matter in what form 

 it may manifest itself, metameric asymmetry in the recent 

 crinoids is an attribute of the most specialized types in 

 the groups in which it occurs. 



From the conditions in the Plicatocrinidne, the last 

 remnants of the once abundant Tnadnnnta, it would appear 

 that asymmetry is an attribute of phylogenetically deca- 

 dent types — types in which type senescence has so far ad- 

 vanced as to inhibit the normal course of development. 



The Geographical Distribution of Asymmetry 

 The geographical distribution of asymmetry is as inter- 

 esting as the phylogenetical distribution. 



Asymmetry of Type 2. 

 Asymmetry of Type 2. 

 Asymmetry of Type 3. 

 Asymmetry of Type 3. 

 Asymmetry of Type 3. 



