POLYPLACOPHORA. XXXI 



mucro elevated, posterior, 



Ischnoplax, p. 64. 

 ff. Shell not greatly elongated. 



g. Girdle clothed with large 

 and small intermingled 

 scales, Heterozona, p. 65. 

 gg. Girdle scales regular, uni- 

 form. 



h. 1 slit on each side of 

 median valves 

 i. Girdle scales flat, 

 Ischnochiton, p. 86. 

 ii. Girdle scales very 

 convex, 



Lepidozona, p. 125. 

 hh. 2 or more slits on 

 each side, 



i. Girdle scales flat, 

 Radsiella, p. 139. 

 ii. Girdle scales con- 

 vex, 



Ischnoradsia, p. 144. 



dd. Anterior and side slits corresponding in 

 position with external ribs (Callistoplacinse). 

 e. Surface of valves having strong radial 

 ribs ; girdle densely clothed with im- 

 bricating scales, 



CALLISTOCHITON, p. 260. 

 ee. Surface of valves granulated or peb- 

 bly ; girdle not densely imbricated 

 with scales. 



f. Anterior valve with more than 

 7 slits ; sinus very spongy ; mu- 

 cro posterior, 



NUTTALLINA, p. 277. 



ff. Anterior valve with 5 slits ; mu- 

 cro subcentral, not posterior, 

 g. No sutural pore tufts. 



CRASPEDOCHITON, p. 285. 



gg. Sutural pore tufts present, 



ANGASIA, p. 286. 



