\X.\ii POLYPLACOPHORA. 



e> f. Surface of valves wrinkled or ribbed ; 

 girdle naked except for hairs or cor- 

 neous spines. 



/. Valves having very strong radial 

 ribs; girdle naked, with sutural 

 tufts, CALLISTOPLAX, p. 288. 

 jj. Valves not very strongly sculp- 

 tured ; girdle tough, with corne- 

 ous curved spine-like processes, 

 CERATOZONA, p. 290. 



Posterior valve having a sinus behind, with one slit 

 or none on each side ; girdle hairy or nude, never 

 scaly (Mopaliida'). See key to genera on page 293. 

 66. Surface of intermediate valves divided into a narrow 

 dorsal area, and latero-pleural areas, the latter formed 

 by the union of the lateral and the pleural (or sides of 

 the central) areas ; valves more or less covered by the 

 naked, spiculose or hairy (never scaly) girdle ; gills gen- 

 erally short (Acanthochitoid phylum). 

 c. Posterior valve either having well developed slits, 

 or a posterior sinus, or both ; not funnel-shaped, 

 anterior valve with 5 or more slits (Acanthochiti- 

 dce, see next volume for species). 



d. Valves not completely covered by the girdle, 

 the first 7 having their apices posterior and 

 marginal. 



e. Anterior valve having 5 regularly 

 placed slits; exposed portion of each 

 median valve much longer than wide. 

 /. Girdle provided with pores bear- 

 ing tufts of bristles ; posterior 

 teeth wanting or irregular 



ACANTHOCHITKS. 



ff. Girdle naked ; posterior valve 

 with 6 slits, the teeth even, spread- 

 ing LEPTOPLAX. 

 jfjfjf. Girdle spongy, produced forward 

 as in Placiphorella 



SPONGIOCHITON. 



