XXX POLYPLACOPHORA. 



illose ; gill-row short or long, 



TRACHYDERMON, p. 67- 

 ee. Valves solid at eaves, generally with 

 beaded sculpture ; teeth sharp, often 

 rugose outside; girdle leathery with 

 few or many hairs or spiculse, never 

 scaly ; gills as long as the foot (Chatse- 

 pleuroid group). 



/. Sinus squared, the sutural plates 

 not connected across it; girdle 

 hairy or naked. 



g. Mucro in front of the mid- 

 dle, CH^ETOPLUERA, p. 27, 

 [and appendix. 



gg. Mucro decidedly posterior, 



PALLOCHITON, p. 256. 



ff. Sutural plates connected across 



the sinus ; girdle thick, bearing 



minute clumps of short spicules, 



DINOPLAX, p. 254. 



eee. Valves solid at eaves ; girdle densely 

 covered with imbricating scales, rare- 

 ly elongated like fluted pillars (Isch- 

 noid group) ISCHNOCHITON. 



/. Shell much elongated, narrow 

 (gills short at both ends ; foot no- 

 tably dilated and produced in 

 front, at least in some of these 

 sections). 



g. Girdle scales uniform ; mu- 

 cro near the middle 



h. Valves longer than 

 wide, roundly arched, 

 with several side slits ; 

 animal Julus-like, 



Stenochiton p. 55. 

 hh. Less elongated ; Amer- 

 ican forms 



Stenoplax, p. 56. 

 gg. Girdle scales very unequal ; 



