THE 



AMERICAN NATURALIST 



Vol. XLIX. September, 1915 No. 585 



A STUDY OF ASYMMETRY, AS DEVELOPED IN 

 THE GENERA AND FAMILIES OF 

 RECENT CRINOIDS 



AUSTIN H. CLARK 



Preface 521 



The Different Types of Criuoid Asymmetry 523 



The Asymmetrical Crinoids 524 



The Phylogenetic Distribution of Asymmetry 526 



Bathymetrical Distribution of the Asymmetrical Crinoids 530 



Thermal Distribution of the Asymmetrical Crinoids 535 



The Asymmetrical Features in Detail 538 



Summary 546 



Preface 



In the animal kingdom there are few, if any, forms 

 which can lie properly described as perfectly symmetrical, 

 either from a bilateral or a radial standard. We have, 

 however, become accustomed to refer to many types as 

 "asymmetrical." In the sense in which we employ this 

 word we do not intend to convey the meaning that these 

 types alone of their respective classes depart from true 

 bilateral or radial symmetry, but rather to indicate that 

 they exhibit more asymmetry than the maximum contem- 

 plated in our generalized concept of, or arbitrary stand- 

 ard for, those classes. 



Thus we readily recognize and confess the asymmetry 

 in the skull of the narwhal (Monodon) with its single 

 greatly elongated and twisted incisor, and the asymmetry 

 in the bones in the skull of the whales, while at the same 

 time we commonly consider man to be symmetrical, 

 though careful measurement shows the right arm and 

 521 



