316 PLAXIPHORA. 



b. Sutural pores or tufts distinctly developed ; sinus shallow. 

 c. Anterior valve distinctly ribbed ; lateral areas dis- 

 tinct, setiger, biramosa. 

 cc. Anterior valve smooth ; lateral areas indistinct. 

 Dark brown, banded with paler at the ridge, 



simplex, 

 bb. Sutural tufts or pores absent. 



c. Lateral areas undefined, not sculptured. 



d. Sinus deep, narrow, denticulate ; brown, with 

 a yellow-bordered dorsal band of chestnut, 



wahlbergi. 



dd. Sinus wide, smooth; black, with a green- 

 bordered dorsal black triangle, glauca. 

 cc. Lateral areas defined by a beaded rib, concentric- 

 ally wrinkled. 



d. Colors bright and variegated ; size small, 



carpenteri. 

 aa. Central areas corrugated, at least at the sides, 



Group of P. petholata. 

 Group of P. terminal/it. 



Group of P. setiger. 



Shell smoothish ; girdle with distinct sutural pore-bunches. 

 P. SETIGER King. PI. 65, figs. 76, 77, 78, 79. 



Shell elliptical^ low roof-shaped, the ridge carinated, side-slopes 

 straight. Surface lusterless, smooth except for delicate growth-lines. 

 Color ashy-olive or bluish, with concentric dark streaks, and gener- 

 ally having unequal stripes radiating from the beak of each valve, 

 these markings varying from indistinct to conspicuous. 



The median valves are not beaked. Lateral areas not raised, but 

 clearly defined by a narrow, raised diagonal riblet, and perceptibly 

 swollen or ribbed toivard the posterior margin, giving the young a 

 bicostate appearance ; the two ribs being more or less nodose, especially 

 in young specimens, and toward the beaks in adults ; and there are 

 also a few extremely indistinct radiating riblets in the slightly 

 excavated middle portion of the lateral areas. Central areas smooth. 

 Anterior valve having about 8 narrow radiating riblets, becoming 

 obsolete as they near the peripheral margin, but each ending in a 



