314 PLAXIPHORA. 



depressed, roof-shaped, the dorsal ridge angular, side slopes slightly 

 convex. Color pale brown, whitish along the middle and at the- 

 edge. Girdle extremely wide in front, narrow behind, dark brown 

 in alcoholic specimens. 



Median valves very wide and short, a trifle beaked in the middle* 

 Lateral areas strongly elevated, the diagonal slope abrupt ; slightly 

 excavated in the middle, obsoletely granulated. Central areas hav- 

 ing rather inconspicuous growth-lines and an obsolete or nearly 

 effaced granulation. Anterior valve very short and broad, having 

 many faint radiating riblets and obsoletely granulated throughout. 

 Posterior valve small, with posterior, slightly elevated mucro, and 

 elevated postero-lateral ribs. 



Interior white. Sinus very narrow, square. Insertion-plates very 

 short, almost subobsolete, that of the anterior valve wide, calloused, 

 having about 20 irregular and excessively shallow notches (hardly 

 slits), obsolete toward the ends ; median valves with 1 slit. Poste- 

 rior valve having a low wide callous ridge, slightly rugose, and in 

 the middle interrupted by a shallow rounded sinus (pi. 65, figs. 73- 

 75.) 



Girdle " very broad anteriorly and narrow posteriorly. It increases 

 gradually from the posterior end to a point opposite the fifth plate, 

 where it suddenly expands into a broad round front, with the 

 breadth one-third greater than the greatest breadth of the shell. It 

 is thick, leathery and scabrous, everywhere closely covered with 

 minute spinules ; the lower surface anteriorly shows many radiating 

 grooves (not distinct in the smaller examples) ; *between these are 

 rows of slightly raised small verrucse, covered with small spinules. 

 The inner edge, or mantle-border, is sharply defined, enclosing an 

 elliptical area around the head and gills, with a well marked poste- 

 rior sinus ; its front edge is divided into about seven digitations, the 

 anterior ones rather long, tapering, and tentacle-like but coriaceous- 

 and covered with fine spinules, like the rest of the marginal mem- 

 brane. Cephalic hood large, broad-lunate; foot relatively small, 

 ovate. Gills numerous (in the largest about sixteen on each side), 

 extending nearly the whole length (more than two-thirds) of the 

 foot, but reaching neither end of it." 



Length of largest specimen (alcholic) 32, breadth 26 mill. ; length 

 of shell 21, breadth 18 mill. Divergence 130. 



N. lot. 40 Or, W. long. 68 54', in 610 fms., of Nantucket Island, 

 and in N. tat. 42 15' 25", W. long. 65 48' 40", in 122 fms., off New 



