54 ISCHNOCHITON. 



Synopsis of Subgenera and Sections. 

 Subgenus I. STENOCHITON Ad. and Ang. 



Very much elongated, valves longer than wide, roundly arched, 

 the median valves having several slits ; unibo subcentral ; girdle 

 covered with minute, smooth imbricating scales. 



Subgenus II. STENOPLAX Cpr. 



Elongated, narrow, the posterior valve large, depressed, with sub- 

 central or posterior umbo. Girdle covered with small crowded 

 striated scales. 



Section Stenoplax s. s. Median valves 1 slit. 



Section Stenoradsia Cpr. Median valves with several slits. 



Subgenus III. ISCHNOPLAX Cpr. 



Elongated, narrow ; posterior valve elevated, the umbo posterior; 

 girdle covered with very minute scales, having conspicuously larger, 

 striated scales mingled among them. 



Subgenus IV. HETEROZONA Cpr. 



Shell like normal Ischnochiton. Girdle bearing small scales 

 with large striated scales intermingled. 



Subgenus V. TRACHYDERMON Cpr. 



Oval ; valves having the sharp teeth of normal Ischnochiton ; 

 eaves narrow, rarely spongy. Girdle having very small smooth 

 scales. 



Section Trachydermon s. s. Median valves having one side slit. 



Section Trachyradsia Cpr. Median valves having more than one 

 side slit. 



Subgenus VI. ISCHNOCHITON (Gray) Cpr. 



Oval or oblong ; valves having sharp smooth teeth and solid 

 eaves. Girdle covered with short flat, generally striated imbricating 

 scales of moderate size. 



Section Ischnochiton s. s. Central valves having one slit. 



Section Radsiella Pils. Central valves having two or several 

 slits. 



