332 PLAXIPHORA. 



and smooth. The girdle has soft hairs, and a few larger, and there 

 is some trace of pores. The posterior slit in the girdle mentioned by 

 Adams seems to be an accidental tear ; H. Adams believes the 

 same. 



Length 15, breadth 10 mill; divergence 133. 



P. OVATA Button. PI. 54, figs. 34-40. 



Shell short-oval, very wide in the middle, rapidly narrowing toward 

 the ends ; moderately elevated, the dorsal ridge carinated, side slopes 

 slightly convex. Surface lusterless, sculptured in "herring-bone 1 " 

 pattern. Color buff along the ridge, pale olive-green on the side- 

 slopes, with a small curved dark spot or two on each valve. 



Median valves broadly v-shaped, the apices of the valves elevated 

 and acute. Lateral areas narrow, a little raised, the diagonal rib 

 sculptured with a series of closely super-imposed v's, their apices 

 directed toward the beak of the valve, one limb of each v direc- 

 ted outward, curving, and forming the sculpture of the pleura, 

 the other limb forming an oblique corrugation of the lateral area 

 (fig. 40). Sutural margin of each lateral area obliquely, coarsely 

 granose. Central areas having a nearly smooth band along the 

 dorsal ridge, and numerous slightly curving and diverging riblets on 

 each side. Anterior valve small, its width about that of one side- 

 slope of the fourth valve, its apex elevated and slightly recurved ; 

 surface having 8 strong ribs, besides those at the sutural margins, 

 the anterior ribs strongest and wider apart; ribs nodose, intervals 

 obliquely corrugated. Posterior valve (figs. 36-39) small, forming an 

 extremely narrow crescent, strongly curved upward at the acute pos- 

 terior mucro, the front ("central") area narrow, concave ; viewed 

 from behind (fig. 37) it presents the appearance of a wide inverted 

 v. 



Interior bluish-white. Sinus exceedingly broad, rounded, the 

 sutural-plates widely separated. Insertion plates long, having the 

 slit near the posterior edge of the plate. Anterior valve having 8, 

 median valves 1 slit. Posterior valve (fig. 39) having a keel-like 

 insertion-plate without slits on each side, and a broad sinus in the 

 middle behind. 



Girdle rather narrow, yellowish, bearing a pore-tuft of hyaline 

 white bristles at each suture and a fringe of bristles at the edge. 



Length 15, breadth 13 mill.; divergence 112. 



Cook Strait; Dunedin, New Zealand ; on seaweed. 



Acanthochcdes ovatus HUTTON, Trans. N. Z. Inst. iv, p. 182 (1872). 



