134 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



PLEUROSIGMA ELONGATUM W. Smith 



(Smith, Brit. Diat., pi. 20, fig. 199; Van Heurck, Synopsis, pi. 18, fig. 7;]Pera- 

 gallo, Pleuro., pi. 3, figs. 5, 7.) 



This is one of several species which De Toni (Syl. Alg., p. 233) 

 unwisely huddles together under P. angulatum. 



PLEUROSIGMA EXEMPTUM, new species 



Plate 29, fig. 7-8 



Valve broadly lanceolate, moderately sigmoid, tapering from the 

 broad middle portion to the somewhat blunt ends; rhaphe more sig- 

 moid than the valve, thereby approaching opposite sides toward the 

 ends; a slight lateral hyaline dilation around the central nodule; 

 markings quincunx, but unusually arranged, in that the customary 

 transverse lines are lacking, and are replaced by faint longitudinal 

 lines. This is due to the angle between the oblique lines being^not 

 the normal 60° but slightly in excess of 90°, so that the beading 

 becomes so spaced that longitudinal lines appear bisecting the obtuse 

 angle. 



Length 0.504-1.16; with 0.09-0.116; lines 10.3-11 in 0.01 mm. 



Although the color of specimens is a very untrustworthy specific 

 character, the almost uniformly heavy coloration of this species is 

 noteworthy. Specimens are very conspicuous not only by reason of 

 their great size but because they are a dark greenish-brown with a 

 strong metallic luster. 



Rather plentiful in most of the Philippine Islands gatherings and 

 especially s > in one from Jolo Jolo, Sulu Islands. 



Type.— Cat. No. 43671, U.S.N.M. 



PLEUROSIGMA FALX, new species 



Plate 30, fig. 1) 



Valve lanceolate, sigmoid, tapering to the narrow but not acute 

 ends; rhaphe more sigmoid than the valve, being plainly oblique to 

 its long axis at the middle, but approaching only slightly the oppo- 

 site sides of the valve toward the two ends; a small lozenge-shaped 

 central area; markings coarse, rigidly quincunx, the angles between 

 the six lines from any given point being 60°. The valves are unusu- 

 ally robust and thick walled, so that the outline appears as a heavy 

 dark line. 



Length, 0.119; width, 0.0194; 15 diagonal lines in 0.01 mm. 



There is some resemblance between this relatively coarsely marked 

 species and small specimens of P. decorum W. Smith, except for the 

 much more sicklelike swing of the latter and the nearer approach of 

 its very sigmoid rhaphe to the sides of the valve. This species is 

 plentiful in some Philippine Islands material and it occurs occasion- 

 ally in Galapagos Islands dredgings. 



Type.— Cat. No. 43672, U.S.N.M. 



