140 ISCHNOCHITON-RADSIELLA. 



chitons, but insertion-plates of the intermediate valves having two or 

 several slits. 



The present section differs from Ischnoradsia Shuttleworth (Lep- 

 idoradsia Cpr.), in having the girdle covered with flat, striated scales, 

 whilst in the latter group the scales are transformed into smooth, 

 solid, pebble-like bodies. Radsiella differs from the Radsioid section 

 Stenoradsia in having the short contour and anterior mucro of the 

 true Ischnoehitons, and in the normal foot and gills. 



Carpenter's use of the name Ischnoradsia Shuttlw. for this group 

 is clearly indefensible, as Shuttleworth mentions none of the 

 species now included herein, whilst his diagnosis covers all Radsioid 

 forms of Ischnochiton. I have therefore restricted the name/sc/mo- 

 radsia to the forms grouping around Shuttleworth's first species, 

 australis. 



I. TRIDENTATUS Pilsbry, n. sp. PL 18, figs. 35, 36, 37, 38, 39. 



Shell small, oval, moderately arched, slate-color with white spots 

 and dots, or white mottled with slate ; the girdle tessellated with 

 alternate dark and light, the interior blue, with a leaden spot under 

 each sutural-plate, and short dark rays from the apex. 



The valves are evenly arched. The posterior valve (fig. 39) has 

 a central, rather depressed umbo, the slope behind it concave. 

 Lateral areas raised a trifle, but scarcely distinguishable; central 

 areas evenly, closely and finely pitted in quincuncial pattern ; the 

 lateral areas are a little more coarsely sculptured with close low 

 diamond-shaped granules ; the granulation generally less regularly 

 arranged on the end valves than on the lateral areas. 



Interior with low, arched sutural-plates and wide, flat, angular 

 sinus; anterior valve with 13, posterior 12, central valves 2 slits. 

 Teeth sharp, smooth. Eaves very narrow. 



Girdle covered with imbricating striated scales (fig. 36.) 



Length 15, breadth 10 mill. 



La Paz, Lower California (Lockington) ; Gulf of California 

 (Gabb.) 



? ? Ischnoradsia dispar CARPENTER, MS. not SOWERBY. 



This species is easily separated from I. dispar Sowb. by the 

 apparently similar sculpture of central and lateral areas, the sculpt- 

 ure in dispar being obviously diverse on these areas. I suppose this 

 to be what Carpenter considered dispar merely because it has 

 Radsioid valves ; but Carpenter has given no description of his shells. 



