THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 73 



GOMPHONEMA PARVULUM VAR. MICROPUS (KUETZ.) CL. 



Valve clavate, with rounded apex and basis; axial area indistinct; central area 

 unilateral, with a small stigma; striae distant in the middle. 

 Common. 

 PI. 19, Fig. 17. 



GOMPHONEMA VENTRICOSUM GREG. 



Valve clavate, with broad apex and produced, rounded basis; axial area narrow, 

 widened in the middle; stigma one; strise distant in the middle, finely punctate. 

 Blue clay. 

 PI. 19, Fig. 13. 



GOMPHONEMA OLIVACEUM LYNG. 



Valve clavate, with broad apex and narrow basis; axial area very narrow; central area 

 irregular, without stigma; strife radiate, finely punctate. 

 Very common. 

 PI. 19, Fig. 23. 



GOMPHONEMA BRASILIENSE VAR. DEMERAR^E GRUN.? 



Valve lanceolate, with sub-cuneate apex and narrowed basis; axial area lanceolate, 

 broad; no stigma; median fissures remote; striae parallel, 12 in 10 n, punctate, the puncta 

 obsolescent, small or interrupted. 



Willistown, Pa. Rare. 



PL 19, Fig. 24. 



PLEUROSIGMA WM. SM. (1852) 

 (pleura, a side, and sigma, the letter s) 



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

 in transverse and oblique lines. 



Cleve divides the forms usually known as Pleurosigma into two genera, Pleurosigma 

 and Gyrosigma. Pleurosigma includes all forms having oblique rows of puncta, while 

 Gyrosigma includes all having longitudinal rows. Both have transverse striae. The former 

 consists entirely of marine species, while in the latter the species are found in fresh, brack- 

 ish and salt water. 



The endochrome in Pleurosigma, according to Mueller, consists of two bands which 

 differ in the median part of each valve. Mereschkowsky says that the endochrome is so 

 divided as to form four bands, two on each valve, that their position is different in different 

 species, and that they are not the same on valves of the same frustule. 



Cleve prefers to classify the species of Pleurosigma and Gyrosigma in accordance with 

 the outline of the valve and the flexure of the median line. I shall, however, retain the 

 method used by Peragallo and Grunow and arrange the forms according to the striation. 



(1) OBLIQUE STRIDE ABOUT 90 DEGREES, MORE DISTINCT THAN THE TRANSVERSE 



PLEUROSIGMA FORMOSUM WM. SM. 



Valve elongated, slender, gently sigmoid, acute at the ends; oblique strise crossing 

 each other at about 90 degrees; 10-16 in 10 n; transverse strise, 14-20 in 10 M (Cleve). 

 Along the coast. 

 PL 22, Fig. 5. 



