POLYPLACOPHORA. XXI 



tition of the group, giving figures and descriptions of the radulse of 

 species representing a majority of the genera of Chitons. In 1889, 

 Dall proposed the superfamily and family names quoted in paren- 

 thesis in the above table. 



The writer proposes to show in the course of this work, that the 

 primary division into Regular and Irregular forms is not funda- 

 mental, the Irregular group being polyphyletic, consisting of genera 

 derived from several totally distinct stocks. With the exception of 

 the Leptoidea (Leptochitonidse), Ischnoidea (Ischnochitonidse) and 

 Chitonelloidea (Cryptoplacidse), all of the family groups of Carpen- 

 ter and Dall require dismemberment. 



DR. PAUL FISCHER, in his Manuel de Conchyliologie, 1885, adopts 

 the Carpenterian arrangement, with some transpositions, but 

 reduces the generic names of former authors to the rank of sub- 

 genera and sections. His scheme is as follows. The Carpenterian 

 equivalents being given in brackets, a glance at the above table will 

 show what groups were included by Fischer. 

 Family CHITONID^E. 

 Genus HOLOCHITON Fischer, 1885 [=Leptoidea Cpr.] 



Subgenus Eochiton Fischer, 1885 [=rPalaeozoic Leptoidea]. 

 Genus CHITON Linn. [Includes Ischnoidea, Lophyroidea, Acanthoi- 

 dea of Cpr.]. 



Subgenus Tomochiton Fischer, 1885 [Includes the Ischnoidea and 

 Acanthoidea ischnoidea which lack girdle pores]. 



Subgenus Porochiton Fischer, 1885 [Includes the Ischnoidea and 

 Acanthoidea ischnoidea which have girdle pores]. 



Subgenus Chiton s. s. [ Lophyroidea of Cpr.]. 



Subgenus Acanthopleura Guild. [= Acanthoidea typica Cpr.]. 

 Genus ANISOCHITON Fischer, 1885 [Includes Schizoidea, Placipho- 



roidea, Mopaloidea of Cpr.]. 



Genus CHITONELLUS Lam. 1819 [= Chitonelloidea Cpr.]. 

 Genus DIARTHROCHITON Fischer, 1885 [Cryptoidea Cpr.]. 



It will be noticed that Fischer ignores the law of priority in the 

 use of generic names. His classification is not otherwise very different 

 from Carpenter's, except that he reduces the genera of Gray and 

 Carpenter to the rank of sections. 



M. A. T. DE ROCHEBRUNE, in several papers upon Chitons, pub- 

 lished from 1881 to 1889, has classified the group in various ways, 

 the principal schemes being given below. It is necessary to give in 

 brackets the equivalents of Rochebrune's genera, in order to under- 



