320 PLAXIPHORA. 



but well indented at the sutures, having a " false apex " in front. 

 Lateral areas having indistinct, subobsolete radiating riblets. Ante- 

 rior valve subangled by 8 radii, which slightly undulate the- 

 peripheral margin ; otherwise nearly smooth except for growth- 

 lines. Interior : posterior valve having the insertion-plate elevated,, 

 rounded, interrupted in the middle behind by a very narrow, deep 

 sinus. Anterior valve having 8, central valves 1 slit ; teeth long, 

 slightly striated and slightly thickened at the edges of the slits out- 

 side. Sinus narrow, short. Sutural laminae continuous over the ante- 

 rior false apex. Girdle (fig. 61) tough ; black, leathery, hardly 

 sinuated behind, smooth, with a few bristles at the sutures and 

 around the margin. 



Length 60, breadth 40 mill. ; divergence 110. 



New Zealand (Mus. Cum., no. 2.) 



This very fine but unsculptured species was marked as new in Dr 

 Gould's collection, but not described by him. It is abnormal in the 

 nearly smooth aspect of the tough girdle, in which however the 

 binary pores (of which five surround the head valve) are distinctly 

 marked. The shape of the valves is nearly as in Mopalia ivosness- 

 enskii. ( Cpr.} 



Professor Hutton has considered his Tonicia corticata a synonym 

 of biramosa, in his Manual of New Zealand Mollusca, 1880 ; but the 

 description (copied below) would hardly lead me to the same 

 conclusion, although Button's species is undoubtedly a Plaxiphora. 

 In sculpture it must resemble P. terminalis. 



Tonicia corticata. Oval ; margin naked ; valves much depressed ; 

 posterior margins slightly concave ; lateral areas bounded on each 

 side by a flatly nodulose ridge, the space between being obliquely 

 striated, the striae running outward and backward ; median areas 

 rugose, without either distinct lines or granules. Length 1*25, 

 breadth 1*13 inch. Color: margin black when dry; valves grey; 

 inside white, covered over with white, coralline growth and small 

 marine algae. Founded on a specimen in the Colonial Museum, 

 locality not stated. (Hutton, in Trans. N. Z. Inst. iv, p. 180, May, 

 1872.) 



P. SIMPLEX Carpenter. PI. 67, figs. 43, 44, 45, 46. 



Shell similar to P. modesta, but more elevated, longer ; valves 

 rude, maculated with brown-purple ; dorsal ridge rounded ; mucro 

 posterior, flattened ; lateral areas scarcely defined ; valves rounded, 



