XXIV POLYPLACOPHORA. 



toward greater definiteness in the positions of the slits, and often 

 toward the degeneration of the shell ; leading (a) through Callisto- 

 placince to the Mopaliidce, and (&) through Acanthoehitidce. to covered 

 forms, Cryptochiton, and to worm-like forms, Cryptoplacidce ; the 

 last having fewer slits than any other Chitons. In the other direc- 

 tion (II) the slits became generally more numerous, the insertion- 

 plates are roughened for the stronger adhesion of the girdle, the 

 valves increase in general effectiveness as a protective armor, and 

 their sense organs became highly developed. This phylum (Chi- 

 tonidce) culminates in Tonicia, Acanthopleura, Schizochiton, Enoplo- 

 chiton, Liolophura, etc. ; and may be regarded as the highest group 

 of Polyplacophora. 



NOTE. It must be understood that in cases where the classifica- 

 tion below given differs from that in the descriptive part of this 

 work, the former represents the mature views of the author. Page 

 23 in the text, should be cancelled, as it was written when but a 

 small portion of the groups there discussed had been studied. The 

 synopses given on pp. 24-25, and 148-149 also require correction. 



Synopsis of Classification of the Order Polyplacophora. 

 I. Superfamily EOPLACOPHORA. 

 Family Lepidopleuridce Pilsbry. 

 Genus Lepidopleurus Kisso. p. 2. 



Section Deshayesiella Cpr., p. 16. 

 Genus Hanleya Gray, p. 17. 

 * 



Genus Hemiarthrum Cpr., p. 19. 

 * 



Genus Microplax Ad. & Ang., p. 21. 



II. Superfamily MESOPLACOPHORA. 

 Family Ischnochitonidce Pilsbry, p. 253. 

 Subfamily Ischnochitoninse Pilsbry, p. 254. 

 Genus Tonicella Cpr., p. 40. 

 Genus Schizoplax Dall, p. 46. 

 Genus Callochiton Gray, p. 48. 



Section Sterochiton Cpr., p. 52. 

 Genus Trachydermon Cpr. p. 67. 



