POLYPLACOPHORA. xvii 



At the same time that Gray published his first <-.s.say, Dr. A. TH. 

 VON MIDDENDORFF proposed an elaborate scheme of classification, 

 in an extensive work upon the Siberian Chitons. His divi.-ion- \v-r- 

 based upon (1) the entirely internal or partly exposed condition of 

 the valves, and position of the apex of each, (2) degree of immer- 

 sion, indicated by the contiguity or separation of the exposed part 

 of the valves, (3) comparative width of the girdle, and (4) presence 

 or absence of girdle-pores. This classification although worked out 

 with vast industry, is a distinct retrogression from the stand taken 

 by de Blainvillein 1825. MiddendorfF however, did excellent work 

 upon the anatomy of the Chitons in this publication. His arrange- 

 ment is as follows : 

 Genus Chiton L. 



(1) Subgenus Cryptochiton Midd. (C. stelleri). 

 (1) Subgenus Phcenochiton Midd. 

 (2) Section Dichachiton Midd. 



(3) Subsection Symmetr ogephyr us Midd. 



(a.) Poriferi, Ch. (Symmetr.) monticularis Q. [=Crypto- 



conchus.] 

 (ft.) Apori, Ch. (Symmetr.) vestitus, pallasi, amiculatus 



[ Amicula.] 

 (3) Subsection Afnetrogephyrus Midd. 



(a.) Poriferi, Ch. fasciatus Q., larvseformis Blv. [=Cryp- 



toplax.] 



(6.) Apori, Ch. Isevis Lm. [ Cryptoplax.] 

 (2) Section Hamachiton Midd. 

 (3) Subsection Platijsemus Midd. 



(a.) Poriferi, Ch. zelandicus, emersoni, biramosus, etc., [ 



Acanthochites, Plaxiphora, etc.] 



(6.) Apori, Ch. submarmoreus, tunicatus, alatus, incisus, 

 wosnessenskii, [ Tonicella, Katharina, Ischnochiton, 

 Mopalia, etc.] 

 (3) Subsection Stenosemus Midd. 



(a.) Girdle hairy, Ch. marmoreus, lineatus, brandti, mercki, 



etc., [^Tonicella, Schizoplax, etc.] , 

 (6.) Girdle finely squamose, Ch. ruber, albus, cinereus, 



etc., [=Trachydermon, Lepidopleurus.] 

 (c.) Girdle coarsely scaly, Ch. mertensii, etc., [ Lepido- 



zona.] 

 MiddendorfFs names being published simultaneously with Gray's, 



