MANUAL OF CONCHOLOGY. 



MONOGRAPH OF THE POLYPLACOPHORA. 



(CONCLUDED.) 



Acanthochitoid Phylum. 



The systematic position and genesis of this stock of chitons has 

 been indicated on pages xxiv and xxvii of the preceding volume of 

 this work. Two families, Acanthochitidce and Cry2)toplacidce, are 

 recognized, both containing some peculiarly modified forms. 



The following diagram expresses the approximate relationships 

 and phylogeny of the genera. It must be remembered that those 

 genera Avhich have undergone the greatest modification are in this 

 phylum distinctly degenerate in character, and represented by but 

 few species of restricted range. 



The reverse is true of the most modified genera of Chitonidce. 



Cryf)tochiton. 



Amicula. 



Cryptoplacidae. 



Choneplax. — 



Acanthochitoid Phylum. 



Cryptoplax. 



