MAEINE DIATOMS OF THE PHILIPPINE ISLANDS 139 



PLEUROSIGMA STRIGOSUM W. Smith 



(Smith, Brit. Diat., pi. 21, fig. 203; H. L. Smith, Type's, No. 414; not Pera- 

 gallo, Pleuro., 5, figs. 1-2.) 



There is not satisfactory ground for De Toni's uniting this with 

 P. angulatum. H. L. Smith's Types, No. 414, is made of material 

 from England, and exhibits on the same slide truly typical specimens 

 of both species. Their points of dissimilarity are easily seen, and 

 William Smith seems to have been warranted in considering the 

 two to be separate species. The two figures in Peragallo's Mono- 

 graphic du genre Pleurosigma are very misleading. 



PLEUROSIGMA SUBRIGIDUM Grunow 



(Peragallo, Pleuro., pi. 2, fig. 3.) 



PLEUROSIGMA SULUENSE, new species 



Plate 30, fig. 6 



Valve barely sigmoid, and only so toward the ends, one side of each 

 half almost straight at the middle and progressively curving toward 

 the end, the other side straight and becoming slighty concave toward 

 the end; rhaphe straight until nearing the ends it approaches the con- 

 cave side, at the middle almost at right angles to the tranverse axis, 

 hooked at its outer ends and without beads at the middle, the two 

 halves being joined around one side of the central nodule; central 

 area slightly dilated transversely; markings uniform over the entire 

 valve, consisting of two sets of oblique lines at an angle of so nearly 90° 

 that no transverse lines are visible, even by oblique light. 



Length 0.529; width, 0.062; 11-11.2 in lines, 0.01 mm. 



So far as the outline is concerned, this diatom is hardly separable 

 from some other species. It very closely resembles the general build 

 of P. majus Grunow (Cleve, Nav. Diat., vol. 1, p. 44, pi. 4, fig. 15), 

 the markings of which are radically different. It is near to what 

 Peragallo, in his Monographic du genre Pleurosigma, plate 2, figure 

 7, calls P. decorum W. Smith, but which does not remotely resemble 

 that species either in form or in marking. (See Smith, Brit. Diat., 

 pi. 21, fig. 19G; H. L. Smith, Types, No. 694; Van Heurck, Synopsis, 

 pi. 19, fig.l, etc.) Diatom literature also contains figures named 

 P. speciosum W. Smith and others P. strigosum W. Smith, which seem 

 to approach closely to the present species; but the typical forms do 

 not at all correspond. In both those species, which are only half or less 

 the size of the present one, the markings are relatively much finer. 

 Thus the average for P. strigosum is length 0.007-0.011 inch, with 

 44 lines in 0.001 inch; while P. suluense is length 0.0208 inch, with 

 27 lines in 0.001 inch. In fact this is a case in which only the com- 

 parison of actual specimens of different species will clearly reveal those 

 adequate specific differences which verbal discriptions and photographs 

 merely suggest. 



Type.— Cat. No. 43677, U.S.N. M. 



