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 system (p. 126). 

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

 Section I. Chitones Eegulares. — Anterior (end ^^osterlor 

 valves of similar character. 



A. Leptoidca. — Insertion plates oljsolete, or, if present, 



unslit ; Lcptochiton, Hanlcyia, Hemiar thrum, Micro2)lax. 



B. Is"h7ioidea. — Insertion plates sharp, smooth, fissured ; 



with eaves ; Trachydernion, Callochiton, Tonicella, Schizo- 

 ])Iax, Leptoplax, Chaetojpleura, Sjoongiochiton, Ischnochiton, 

 Callistochiton. 



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



J 



Fig. 264.— Girdles of 

 various Cliitouidae. 

 A, Radsia sulcata 

 Wood, X 2 ; B, 

 Maugeria granu- 

 lata Gmel., x 3 ; 



C, Enoplochiloii 

 niger Barnes, x 3 ; 



D, Acanthochitoyi 

 fascicular is L., x 

 4 ; E, Tonicia fas- 

 tigiata. Sowb., x 4. 



ing backward ; Chitoyi, Tonicia, Eudoxochiton, Craspcdo- 

 chiton. 



D. Acanthoidea. — Insertion plates thrown forward ; Sclcro- 



chiton, Acanthopleura, Dinoplax, Middendorffia, Nuttallina, 

 Arthur ia, Phacellopleura. 

 Section II. Chitones Irregulakes. — Posterior valve 

 ahnormal, or with a sinus behind. 



E. Schizoidea.—Vo&tevioY valve fissured ; Lorica, Schizochiton. 

 ¥. Flaciphoroidea. — Posterior valve unslit, internally ridged, 



umbo nearly terminal ; Enoplochiton, Ornithochiton, 

 Plaxiphora. 

 G. Mopaloidea. — Posterior valve with posterior sinus and one 

 slit on each side ; 2Iopalia, Katherina, Acanthochiton, 

 Notoplax. 



