THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 75 



(4) OBLIQUE STRIDE 60 DEGREES, THE TRANSVERSE MORE DISTANT 

 PLEUROSIGMA RIGIDUM WM. SM. 



Valve nearly straight or slightly sigmoid, with obtuse ends; raphe central, excentric 

 near the ends; oblique striae, 17-21, transverse, 16-19 in 10 p.. (Peragallo). 

 New Rochelle, N. Y. 

 PL 22, Fig. 2 (very near the var. gigantea Grun.) 



GYROSIGMA HASSALL (1845) 

 (gyros, curved, and sigma) 



Valve lanceolate, sigmoid; axial area very narrow, central area small; strise punctate, 

 in transverse and longitudinal rows. 



Chromatophores two, in long and narrow bands, perforated, differing from those of 

 Pleurosigma. The elaeoplasts are also arranged differently in the two genera. (Meresch- 

 kowsky, Etudes sur 1'Endochrome des Diatomees, Imperial Academy of Petrograd, 1901, 

 Vol. 11, No. 6, p. 18 et seq.) 



The arrangement is according to Peragallo. 



(1) LONGITUDINAL STRIDE MORE DISTANT THAN THE TRANSVERSE 



GYROSIGMA HIPPOCAMPUS (EHR.) 



Valve lanceolate, sigmoid, with obtuse ends; raphe nearly central; transverse strise 

 15-17, longitudinal, 10-12 in 10 p.. 

 Navicula hippocampus Ehr. 

 Pleurosigma hippocampus (Ehr.) Wm. Sm. 

 Gyrosigma attenuatum (Kuetz.) Cl. 

 Long Island Sound. 

 PL 23, Fig. 3. 



(2) LONGITUDINAL AND TRANSVERSE STRIDE NEARLY EQUAL 

 GYROSIGMA BALTICUM (EHR.) CL. 



Valve with margins parallel nearly to the extremities, which are suddenly unilaterally 

 sub-conical and obtuse; raphe sigmoid; transverse and longitudinal strise nearly equally 

 distant, 15 in 10 p. (Per.). L. 200-360 p. 



Navicula baltica Ehr. 



Pleurosigma balticum (Ehr.) Wm. Sm. 



Common along the coast. 



PL 23, Fig. 2. 



GYROSIGMA PARKERI VAR. STAURONEIOIDES GRUN. 



Valve lanceolate, slightly sigmoid, ends produced into beaks with sub-acute apices; 

 raphe straight in the middle part; central nodule elliptical; transverse striae, 21, and longitu- 

 dinal, 24 in 10 p. (Per.). 



An apparent stauros, variable in width, extends to the margin and, in consequence, 

 the median transverse strife are more evident. L. 75 p.. 



Schuylkill River. Rather rare. 



PL 23, Fig. 7. 



