POLYPLACOPHORA. XXX111 



ee. Anterior valve having the slits irregu- 

 larly spaced, more than 5 ; posterior 

 valve with a median angular tail sinus ? 

 its edge with several unequal notches 

 or slits ; exposed part of valves flask- 

 shaped KATHERINA. 

 eee. Anterior valve having 6-8 unequal 

 slits; posterior valve 2 slits and a 

 median wave ; exposed part of each 

 valve far shorter than the ridge of the 

 valve, heart-shaped AMICULA. 

 dd. Valves completely covered by the girdle, 

 their apices not marginal, CRYPTOCHITON. 

 b. Median valves partly lacking side slits ; tail valve with 

 2 or no slits 



cc. Posterior valves without distinct slits, and having 

 no posterior sinus ; more or less hollowed out and 

 funnel-shaped ; anterior valve with 3-5 slits ; body 

 rather vermiform ; gills short, posterior ( Cryptopla- 

 cidce ; see next volume for species). 



d. Valves small, narrow and sagittate, gener- 

 ally partly disjointed ; not covering the 

 body to any considerable extent 



CRYPTOPLAX. 



dd. Valves transverse,'all strongly imbricating ; 

 covering a considerable portion of the upper 

 surface CHONEPLAX. 



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

 teeth by slits ; the teeth sharply sculptured or "pectinated " out- 

 side by fine vertical grooves (Chitonidce). 

 b. Valves lacking eyes (Chitonince). 



c. Girdle scaly CHITON, p. 149. 



cc. Girdle leathery, with short bristles 



EUDOXOCHITON, p. 192. 



bb. Valves having eyes ; posterior valve not deeply sinused 

 behind, its insertion plate developed (Toniciince) 



c. Girdle leathery, nude or nearly so, TONICIA, p. 194. 

 cc. Girdle covered with calcareous spines or spinelets, 



ACANTHOPLEURA, p. 213. 



bbb. Valves having eyes (rarely subobsolete) ; posterior 



