Polyplacophora.] 



AMPHINEURA. 



KEY TO GENERA. 

 a. Valves lacking insertion plates 



an. Valves possessing insertion plates, valves i-vii or i-viii 

 having slits ; teeth smooth or but slightly roughened 

 between the slits, never closely, finely pectinated. 

 Valves lacking eyes (except Callochiton). 

 b. Surface of intermediate valves divided into lateral 

 and central areas by a diagonal (often indistinct) 

 extending from beak to outer front angle of teg- 

 mentum ; or, if this is not clearly the case, the 

 posterior valve has an even, crescent ic series of 

 well-developed teeth ; all valves having slits. 

 c. Posterior valve having crescentic series of well- 

 developed teeth. 



d. Valves porous at the eaves. Sutural plates 

 connected across the sinus, side slits 

 several (single in one species), girdle 

 with compact diamond - patterned 

 covering ; valves with minute eyes . . 

 dd. Valves solid at eaves, girdle densely 

 covered with flat imbricating scales, 

 side slits single 



cc. Posterior valve having a sinus behind, with one 

 slit or none on each side ; girdle hairy or 

 nude, never scaly. 

 d. Posterior valve having a slit on each side of 



the median sinus 

 dd. Posterior valve having a median tail-sinus, 



but no slits 



bb. Surface of intermediate valves divided into a nar- 

 row dorsal area and latero-pleural areas, the 

 latter formed by the union of the lateral and 

 pleural areas ; valves more or less covered by 

 the naked spiculose or hairy (never scaly) girdle. 

 c. Girdle provided with pores bearing tufts or 



bristles 

 cc. Girdle spongy, produced forward 



aaa. All valves, or valves i-vii, possessing insertion plates 

 cut into teeth by slits ; teeth sharply sculptured or 

 " pectinated " outside by fine vertical grooves. 

 6. Valves lacking eyes. 

 c. Girdle scaly 



cc. Girdle leathery, with short bristles 

 bb. Valves having eyes ; posterior valve not deeply 



sinused behind, its insertion plate developed. 

 c. Girdle leathery, nude or nearly so . . 

 cc. Girdle covered with calcareous spines 

 bbb. Valves having eyes ; posterior valve having a deep 

 sinus behind, or lacking the insertion plate alto- 

 gether. No eyes on head valve, its ribs not cor- 

 responding to slits ; girdle densely scaly, slit 

 behind 



tibbb. Valves having eyes upon the lateral areas and 

 head valve. Insertion plate of tail valve re- 

 duced to a smooth ledge or ridge, having no 

 posterior sinus. Girdle leathery, microscopic- 

 ally velvety 



LEPIDOPLEURUS. 



CALLOCHITON. 

 ISCHNOCHITON. 



MOPALIA. 



PLAXIPHORA. 



AC'ANTHOCHITES. 

 SPONGIOCHITON. 



CHITON. 

 EUDOXOCHITON. 



TONICIA. 

 ACANTHOPLEURA. 



LOEICA. 



ONITHOCHITON, 



