MARINE DIATOMS OF THE PHILIPPINE ISLANDS 137 



ippine Islands. Not only does Castracane correctly figure this with 

 only two sets of lines, both oblique, but Peragallo in his Monographie 

 du genre Pleurosigma, plate 3, figure 15, adds a question to the draw- 

 ing of the transverse lines. The present species is equally destitute 

 of transverse lines and its two oblique sets of lines are at 90° to each 

 other. An even further departure from the normal quincunx mark- 

 ing is found in my P. exemptum (q. v.) where the upper and under 

 angles between the oblique lines are actually in excess of 90°, so that 

 a faint suggestion of a third set of lines, not transverse but longitudi- 

 nal, is discoverable. It is evident that beads arranged in strictly 

 transverse and longitudinal rows will give rise to only two sets of 

 lines, which will stand at right angles to each other; that a strictly 

 quincunx order will exist only when the three sets of lines subtend 

 angles of 60° between each other, and that when the angle between 

 the two sets of lines becomes greater than 90° a third set of lines 

 will begin to appear as bisecting this wider angle. In the typical 

 P. angulatum the upper and under angles between the two oblique 

 lines is sometimes a trifle less than 60° and the four side-angles pro- 

 portionally more than 60°, with the result that the transverse lines 

 are then more easily seen than the oblique ones. 

 Type.— Cat. No. 43073, U.S.N. M. 



PLEUROSIGMA OBTUSUM, new species 



Plate 30, figs. 2-3 



Valve slightly sigmoid and only slightly tapering, with broad, 

 blunt, rounded ends; markings strictly quincunx, so that the two 

 oblique and the transverse lines are equally visible and subtend an- 

 gles of 60°; rhaphe at the middle oblique to the long axis, at first 

 barely, then more rapidly curved as it approaches the ends, where it 

 is sharply curved, and approaches close to the more sigmoid side of 

 the valve. 



Length, 0.126; width, 0.029; 14-15 lines in 0.01 mm. 



This species stands nearest to my P. prisma (q. v.). Rare. 



Type.— Oat. No. 43674, U.S.N.M. 



PLEUROSIGMA PRISMA, new species 



Plate 30, fig. 4 



Valve broad prismatic, only slightly sigmoid in outline, with very 

 blunt, rounded ends; rhaphe slightly oblique to the long axis at the 

 middle of the valve, perfectly straight until near the apices, then 

 strongly curved, its apical beads elongated and close to the blunt 

 ends, its central beads well separated; a relatively large transversely 

 widened hyaline central area; markings quincunx, the diagonal lines 

 more evident than the transverse. 



