PREFACE. 



The present volume contains a monograph of the important and 

 extensive Order Polyplacopliora, commonly known as Chitons. 

 Although represented by many species in all seas, the Chitons have 

 been much neglected by Conchologists. This may be due to the 

 fact that their characters are not generally understood, or even seen 

 in ordinary cabinet specimens ; and again, no illustrated work treat- 

 ing the group scientifically has hitherto been produced. Certain it 

 is, that if the serious study of this group be once begun, its attrac- 

 tions will be found to equal or excel those of any other families. 

 The numberless modifications of form, the beautiful adaptation of 

 structure to life-habits and station, the marvelous parallel develop- 

 ments, producing almost identical structures in totally diverse- 

 phyla, all distinguish the Polyplacophora as preeminently interest- 

 ing among mollusks. 



The author begs those who may use this work to begin by taking 

 apart one specimen of each species to be studied ; then find the genus- 

 by the Key on page xxviii. After a certain amount of practice the 

 genus may be ascertained by removing valves vii and viii only ; but 

 guessing it from the external features is in most cases uncertain 

 and unscientific. As a rule, Chitons tell no secrets to those who 

 will not take the trouble to disarticulate them. 



It is hoped that describers of new species will hereafter state the 

 generic characters of their novelties. It is as rational to describe a 

 bivalve without examining its hinge-teeth, as to notice the outside 

 only of a Chiton. 



The synonymy is believed to be practically exhaustive, except 

 that useless references to mere lists giving no new information are 

 omitted. 



Attention should here be directed to the new theories of the origin 

 of important structures of shell and girdle, given on pp. ix, xii ; to 

 the original system of classification (p. xxiii), and the phytogenies 

 (pp. xxvii, xxviii). 



The new species described herein which are credited to Carpenter 

 rest upon his types when the descriptions are quoted from his Ms. (as 

 in the case of Callochiton princeps, p. 50). In cases where I have 



