POLYPLACOPHORA. XXIX 



bb. Valves partly covered, the exposed portions small, sep- 

 arated MICROPLAX, p. 21. 

 aa. All valves possessing insertion plates ; valve i, i-vii or i-viii hav- 

 ing slits ; teeth smooth or but slightly roughened between the slits, 

 never closely, finely pectinated ; valves lacking eyes. 



b. Surface of intermediate valves divided into lateral and 

 central areas by a diagonal (often indistinct), extend- 

 ing from beak to outer front angle of tegmentum ; or if 

 this is not clearly the case, the posterior valve has an 

 even, crescentic series of well developed teeth; all 

 valves having slits. 



c. Posterior valve having a crescentic series of well 

 developed teeth (Ischnochitonidce'). 



d. Anterior and side slits not corresponding in 

 position to external ribs (Ischnochitonince.} 

 e. Valves porous at the eaves, appar- 

 ently smooth outside or nearly so; 

 teeth sharp, smooth; girdle naked, 

 sparsely hairy or densely covered with 

 elongated corneous bodies, the visible 

 ends of which appear like diamond- 

 shaped scales (Tonicelloid group). 

 f. Sutural plates not connected 

 across the sinus ; girdle naked ; 

 side-slits single ; gills shorter than 

 the foot. 



g. Median valves having a me- 

 dian slit filled with carti- 

 lage, SCHIZOPLAX, p. 46. 

 gg. Median valves normally cal- 

 cified, TONICELLA, p. 40. 

 ff. Sutural plates connected across 

 the sinus, or side-slits several, or 

 both ; girdle either sparsely hairy, 

 naked, or with compact diamond- 

 patterned covering ; gills as long 

 as the foot, CALLOCHITON, p. 48. 

 (This includes the typical forms 

 of Trachyradsia, p. 83.) 

 fff. Girdle with chaffy scales or pap- 



