POLYPLACOPHORA 



403 



A description has already been given of the dorsal eyes in 



Chiton (p. 187), the nervous system (p. 202), the branchiae (p. 



154), the radula (p. 228), and the generative S3^stem (p. 126). 



The recent Chitons are thus classified by Dr. W. H. Dall : — 



Section I. Chitones Regulaees. — Anterior and posterior 



valves of similar character. 



A. Leptoidea. — Insertion plates obsolete, or, if present, 



unslit; Lepto chiton., Hanleyia^ Hemiarthrum^ Microplax. 



B. Ischnoidea. — Insertion plates sharp, smooth, fissured; 



with eaves ; Trachydermon., Oallo chiton^ Tonicella, Schizo- 

 plax., Leptoplax., Qhaetopleura^ Spongio chiton., Ischnochi- 

 ton^ Callistochiton. 



C. Lophyroidea. — Insertion plates broad, pectinated, project- 



FiG. 264. — Girdles of 

 various Chitonidae. 

 A, Radsia sulcata 

 Wood, X 2 ; B, 

 Maugena granu- 

 lata Gmel., x 3 ; 



C, E nop I o chiton 

 niger Barnes, x 3 ; 



D, Acantho chiton 

 fascicularis L., x 

 4 ; E, Tonicia fas- 

 tigiata Sowb., x 4. 



ing backward ; Chiton^ Tonicia., Eudoxo chiton., Craspedo- 

 chiton. 



D. Acanthoidea, — Insertion plates thrown forward; Sclero- 



chiton., Acanthopleura., Dinoplo.x, Middendorffia., Nuttal- 

 lina, Arthuria., Phacellopleura. 

 Section II. Chitones Irregulaees. — Posterior valve 

 abnormal., or with a sinus behind. 



E. Schizoidea. — Posterior valve fissured; Lorica^ Schizochiton. 



F. Placiphoroidea. — Posterior valve unslit, internally ridged, 



umbo nearly terminal ; Enoplo chiton., Ornitho chiton., 

 Plaxiphora. 



G. Mopaloidea. — Posterior valve with posterior sinus and 



one slit on each side ; 3Iopalia, Katherina, Acanthochi- 

 ton, Notoplax. 



