POLYPLACOPHORA. XV11 



At the same time that Gray published his first essay, Dr. A. TH. 

 VON MIDDENDORFF proposed an elaborate scheme of classification, 

 in an extensive work upon the Siberian Chitons. His divisions were 

 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 ogephyrus Midd. 



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

 conchus.] 

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



(3) Subsection Ametrogephyrus Midd. 



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



toplax.] 



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

 (2) Section Hamachiton Midd. 

 (3) Subsection Platysemus 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, 



