MARINE DIATOMS OF THE PHILIPPINE ISLANDS 131 



PLAGIOGRAMMA POLYGIBBUM Cleve and Grove 



(Le Diat., vol. 1, pi. 8, figs. 7-11; Schmidt, Atlas, pi. 211, figs. 2-5.) 



Here also belong P. quadrigibbum Brun (Schmidt, Atlas, pi. 211, 

 fig. 1) and P. approximatum A. Schmidt (Schmidt, Atlas, pi. 211, 

 fig. 7). 



PLAGIOGRAMMA SULCATUM Cleve and Grove 



(Le Diat., vol. 1, pi. 8, figs. 1-3; Schmidt, Atlas, pi. 210, fig. 5.) 



This departs so widely from other species of Plagiogramma as to 

 awaken doubt of its belonging here. 



PLAGIOGRAMMA TESSELATUM Greville 



(Micro. Journ., 1859, pi. 10, fig. 7; Schmidt, Atlas, pi. 209, figs. 42-50.) 



Castracane's Glyphodesmis murrayana (Chall. Exp., pi. 18, fig. 12) 

 belongs here. 



Genus PLEUROSIGMA W. Smith 



This genus stands out with unusually sharp distinctness, if we in- 

 clude in it Donkinia Raffs, Rhoicosigma Grunow, and Toxonidia Don- 

 kin, which in fact are nothing more than subgeneric divisions of 

 Pleurosigma, but are accorded generic rank to facilitate identification. 

 All are characterized by a more of less sigmoid outline, both as to the 

 contour of the valve and the shape of the rhaphe; and all are strik- 

 ingly alike in the simplicity and elegance of marking that covers the 

 entire valve — a delicate network of closely appressed beading, 

 arranged in one of three ways — in transverse and longitudinl rows, 

 in decussating rows of three directions, with a divergence of approxi- 

 mately 60°, one being transverse and the other two oblique, and 

 thirdly, in oblique rows of two directions with a divergence of approx- 

 imately 90°, there being no transverse arrangement of rows. This 

 last has not been recorded; but a careful inspection of some species 

 grouped with the three-row decussating type shows that the angle 

 between the lines is 90° with no trace of a transverse arrangement. 

 A notable example of this is P. japonicwn Castracane, in the Report 

 of the Challenger Expedition, page 38, plate 29, figure 14, correctly 

 illustrated as having only two sets of lines, though the fact is not 

 commented upon. This species is recorded below, being not uncom- 

 mon in the Philippine Islands, and invariably shows only two rows by 

 oblique as well as by direct lighting. The same is true of my new 

 species P. obesum, under which is discussed the reason for the disap- 

 pearance of the third line. The attempt to divide Pleurosigma proper 

 into two genera — Pleurosigma for species with decussating rows of 

 three directions and Gyrosigma for species with two direction lines, 

 transverse and longitudinal — has nothing to comment it. Not only 

 would it necessitate a third genus to accommodate those forms above 

 mentioned, which have neither of these arrangements of beading, but 



