XXXIV POLYPLACOPHORA. 



valve either having a deep sinus behind, or lacking the 

 insertion plate altogether (Liolophurince). 



c. Median valves having eyes developed only on the 

 diagonal ribs. Sinus notably narrow and small. 

 Insertion plates of tail valve a low, rounded, 

 rugose ridge more or less notched, slit or waved 

 upward in the middle behind, or sharp, long and 

 pectinated. 



d. Eyes developed on radiating ribs of all 

 valves ; the ribs of head valve correspond- 

 ing to internal slits ; girdle having minute 

 spines ; shell elongated, posterior valve and 

 girdle slit behind SCHIZOCHITON, p. 234. 

 dd. No eyes on head valve, its riblets not corre- 

 sponding to slits ; girdle densely scaly 



LORICA, p. 236. 



cc. Eyes developed upon the lateral areas. Sinus 

 wide and large. Insertion plate of the tail valve 

 reduced to a smooth ledge or ridge, having no 

 posterior sinus or wave. 



d. Valves polished or eroded, dark brown out- 

 side and within, having excessively minute 

 eyes scattered over lateral areas and head 

 valve (when not eroded) ; interior very 

 densely, minutely and peculiarly grooved 

 and punctured. Girdle fleshy, bearing sep- 

 arated rude scales, ENOPLOCHITON, p. 252. 

 dd. Valves polished, colored outside, porcella- 

 nous and smooth within, having the eyes 

 disposed in radial bands on each lateral 

 area and the head valve. Girdle leathery, 

 microscopically velvety, 



ONITHOCHITON, p. 244. 



ddd. Valves lusterless, granulated, having minute 

 eyes scattered over the lateral areas, sides 

 of the central areas, and head valve. Gir- 

 dle densely covered with stout calcareous 

 spines LIOLOPHURA, p. 239. 



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