PLAXIPHORA. 325 



P. CAEPENTERI Haddon. PL 67, figs. 33, 34, 35, 36. 



Shell smooth, the sides meeting at an angle of 105. Anterior 

 valve with eight radiating ridges, not counting the swollen posterior 

 borders. Numerous concentric minor ridges cause these ridges to be 

 rugose. Under surface with nine slits, two being very close together ; 

 eaves short. 



Intermediate valves: central area smooth, with fine lines of 

 growth ; no distinct keel. Lateral areas prominent, concentrically 

 rugose, with a beaded ridge along the anterior border, and, to a less 

 extent, along the posterior border. Under surface with a median hori- 

 zontal rib-like swelling; sutural laminae broad, but not deep ; jugal 

 sinus shallow ; one lateral slit ; eaves short. Posterior : valve anterior 

 area so large as to reduce the posterior area to a minimum ; smooth ; 

 umbo minute, nearly terminal. Posterior area merely forms a con- 

 centrically grooved thickening of the posterior border of the valve- 

 Under surface : sutural laminae as before, but the jugal sinus is com- 

 paratively narrow and deep ; slits and teeth absent ; the posterior 

 border is greatly swollen. 



Girdle closely beset with very minute horny spines. The specimen 

 had been dried when I received it, and none of the longer spines, 

 characteristic of other species, are observable ; but I think I can 

 discern traces of small sutural tufts. 



Color : anterior valve with a median triangular area, with the 

 apex at the umbo, is of a kind of chocolate color, which is variegated 

 with minute zigzag grey lines ; the lateral spaces are of a dark slate 

 color. Intermediate valves: central area: jugum pink; greenish 

 toward the umbo when worn down. Pleura dark greenish-brown, 

 with irregular longitudinal green bands. Lateral areas madder 

 brown, blotched with brown. Posterior valve : anterior area resem- 

 bling the central areas ; posterior border pinkish. Under surface of 

 all the valves pale bluish green ; laminae of insertion white. Girdle 

 reddish-brown. 



Length 14, breadth 9 mill. ; height 4 mill. (Haddon.') 



Tristan da Cunha. 



Plaxiphora carpenteri HADDON, Challenger Rep. Polyplac., p. 34, 

 t. 1, f. 8; t. 3, f. Sa-g. 



The sculpture of the shell, slight as it is, is sufficient to character- 

 ize this species. (Haddon.*) 



P. GLAUCA Quoy & Gaimard. PI. 68, figs. 68-72. 



Animal oval, widened at the sides. Valves wide, rounded, little 



