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THE AMERICAN NATURALIST [Vol. XLIX 



The Phylogenetic Distribution of Asymmetry 



The phylogenetic distribution of the asymmetry among 

 the recent crinoids is very interesting. 



Asymmetry is almost universal in the comatulid family 

 Comasteridae, which includes the most specialized of all 

 recent forms; in this family the first and second types 

 occur, though the latter is much less common. 



Asymmetry is characteristic of the genus Promacho- 

 crinus, which is probably rightly considered as the most 

 specialized genus in the subfamily Heliometrinae, the 

 largest and most universally distributed subfamily of the 

 at present dominant family Antedonidae; in the genus 

 Promachocrinus the first and third types occur. 



Asymmetry is equally characteristic of the genus Thau- 

 matocrinus, the most specialized genus of the family 

 Pentametroerinidae ; in this genus the third type is found. 



Asymmetry exists in all of the genera of the Plicato- 

 crinidae, which includes the last highly specialized expo- 

 nents of the ancient order Inadunata, which nourished 

 from the Ordovician to the Carboniferous, with one 

 family extending into the Permian and Trias and another 

 (the present family) appearing in the Jura ; in the Plicato- 

 crinidae the first, second and fourth types occur in recent 

 genera, while the third is also found in fossil genera. 



Asymmetry is characteristic of both of the recent gen- 

 era of Apiocrinida', which are the most specialized genera 

 in the family; in these the second type occurs. 



Asymmetry of the second type is characteristic of the 

 only recent genus of the Holopodidae. 



Asymmetry characterizes both of the species of Rhizo- 

 crinus — which is at least as highly specialized as any of 

 the genera of the Bourgueticrinidae — existing in the pres- 

 ent seas, and one of the species of Monaclincrinus, a genus 

 of which the exact phylogenetic position is uncertain, 

 although it is probably on a par with Rhizocrinus; in 

 these the third type occurs. 



In the following list the recent asymmetrical types are 



