138 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



Length, 0.129-0.130; width, 0.031-0.035; 13 lines in 0.01 mm. 

 This is one of the most heavily built and coarsely marked of the 

 quincunx Pleurosigmae. 



Type.— Cat. No. 43675, U.S.N. M. 



PLEUROSIGMA RHOMBEUM Grunow 



(Peragallo, Pleuro., pi. 3, figs. 13-14.) 



Pound previously at Samoa and at Auckland, New Zealand. 



PLEUROSIGMA RIGENS, new species 



Plate 30, fig. 5 



Valve prismatic or spindle-shape, with straight sides, not sigmoid 

 except that the blunt ends tend very slightly to opposite sides and 

 the rhaphe somewhat more so; the valve narrows from middle to 

 near the ends, then the sides become parallel; markings quincunx, 

 the angle between the oblique lines a trifle less than 60°, so that the 

 transverse lines are more prominent than the oblique; rhaphe per- 

 pendicular to transverse axis at the middle; the central area expanded 

 laterally into a small oval; markings about this area invariably 

 rugose because of irregularly spattered blotches somewhat larger than 

 the beading, so that the center of the valve always appears to be 

 dirty. 



Length, 0.094-0.127; width, 0.020-0.022; 16-17 oblique lines in 

 0.01 mm., 15 transverse lines in 0.01 mm. 



Although this might be classified as a wide variety of P. rigidum 

 W. Smith, I suggest a new name, because I find it to be very uniform 

 in its much smaller size and its relatively coarser markings than that 

 species. It is abundant in most of the Philippine Islands dredgings, 

 including the Sulu group, and is at times associated with P. rigidum, 

 from which it is readily distinguished by its deep brown color, its 

 minuteness, and the blotched appearance of the valve around the 

 central nodule. The known measurements of P. rigidum are length 

 0.30-0.44; width 0.04-0.068; 18-20 oblique lines, 17-19 transverse 

 lines in 0.01 mm., which contrast strongly with those of this species. 



Type.— Cat. No. 43676, U.S.N.M. 



PLEUROSIGMA RIGIDUM W. Smith 



(Smith, Brit. Diat., pi. 20, fig. 198; H. L. Smith, Types, No. 410; Peragallo 

 Pleuro., pi. 6, figs. 3-6.) 



Rather abundant and showing several variations from the type. 



PLEUROSIGMA SIMILE Grunow 



(Peragallo, Pleuro., pi. 7, fig. 27.) 



The suggestion of Peragallo that this may be a variety of 

 P. balticum W. Smith can not be commended. 



