290 



THETIS SCIENTIFIC RESULTS. 



PELECYPODA. 



FamUy NUCULID^. 



PRONUCULA, gen. nov. 



This genus differs from Nucula by having the hinge line arched 

 instead of angulated, the rows of teeth do not meet or overlap 

 beneath the umbones, but are distant from the chondrophore, 

 which is not oblique as in Nucula, but perpendicular. Briefly, 

 the constituents of the hinge, which in Nucula are much com- 

 pressed and perhaps slightly rotated, are here wide spread. The 

 shell has not the trigonal contour of Nucula, is far thinner and 

 the radial sculpture more pronounced than in that genus. 

 Neither lunule nor escutcheon are present. 



Type Pronucula decor osa, Hedley. 



PRONUCULA DECOROSA, sp. nov. 

 (Fig. 39.) 

 Station 49. 



Shell small, ovate-oblong, moderately inflated, very inequi- 

 lateral, thin, closed, covered by a dense, tough, brown epidermis, 

 on which numerous pustules are irregularly arranged. Beneath 

 the epidermis the shell is white, glossy and brilliantly nacreous. 

 Sculpture : until about one-third grown, the shell is rather smooth; 



after that stage, about 

 thirty fine radiating 

 riblets arise and tra- 

 verse the whole valve, 

 except a small an terior 

 and posterior space ; 

 these are ovei--ridden 

 by fine close concen- 

 tric hair lines, which 

 extend the whole 

 length of the valve. 

 Ventral margin 

 rounded, denticulate 

 by the radial riblets. 

 Beak prominent, bear- 



''""MiM. 



PrOXIX-ULA DECOROSA. 



Fig. 39. 



