74 THE DIATOMACELE OF PHILADELPHIA AND VICINITY 



PLEUROSIGMA OBSCURUM WM. SM. 



Valve linear, not sigmoid, or scarcely so; ends obtuse, subconical; raphe sigmoid, near 

 the margin at the extremities; transverse and oblique striae equidistant, 28 in 10 n (Wm. 

 8m.). 



Abundant at Greenwich Point, Philadelphia. 



PI. 22, Fig. 4. 



(2) OBLIQUE STRIDE CLOSER Ax THE ENDS 



PLEUROSIGMA NAVICULACEUM BREB. 



Valve lanceolate, slightly sigmoid at the extremities; raphe strongly sigmoid near the 

 margin at the ends; central nodule large, rounded; oblique striae, 13-14 in the middle, closer 

 at the ends; transverse stria?, 18-20 in 10 n (Peragallo). 



Long Island Sound. 



PI. 22, Fig. 6. 



PLEUROSIGMA VIRGINIACUM H. L. SMITH 



Valve slightly sigmoid, with acute ends; raphe more sigmoid than the valve, excen- 

 tric near the ends; oblique striae in different directions at the centre, 13 in 10 /j, closer and 

 less distinct at the ends; central nodule small but prominent because of its thickness, 

 producing by diffraction an apparently wide area (somewhat exaggerated in the figure). 

 L. 95 n, usually larger. 



P. affine var. fossilis Grun. (Peragallo). 



P. normanii var. fossilis Grun. (Cleve). 



Common in the blue clay. 



PI. 22, Fig. 8. 



(3) OBLIQUE STRLE 60 DEGREES 

 PLEUROSIGMA ANGULATUM (QUEKETT) CL. 



Valve rhomboidal, with sub-rostrate or produced ends; central nodule rhomboidal; 

 raphe central; transverse and oblique striae at an angle of 60 degrees, equidistant, 18-22 

 in 10 M- 



Navicula angulata Quekett. 



Along the coast. 



PI. 22, Fig. 3. 



PLEUROSIGMA STRIGOSUM WM. SM. 



Valve lanceolate, with sub-acute, somewhat revolute, apices; oblique striae at an angle 

 of about 60 degrees, otherwise as in angulatum. 

 Along the coast. Not common. 

 PI. 22, Fig. 1. 



PLEUROSIGMA ^ESTUARII BREB. 



Valve lanceolate, with produced apices; raphe less sigmoid than the valve and excen- 

 tric; oblique striae, 19-21 in 10 /*, at an angle of about 60 degrees. 

 Along the coast. Common. 

 PI. 22, Fig. 7. 



