104 THE DIATOMACEJE OF PHILADELPHIA AND VICINITY 



PINNULARIA VIRIDIS NITZSCH 



Valve linear-elliptical, with rounded ends; axial area narrow, widened in the middle; 

 striae, 6 to 7 in 10 n, crossed by a band as wide as one-third the length of the striae. 

 Common in fresh water. 

 PI. 29, Fig. 2. 

 Quite variable in size. Approaches P. major by intermediate forms as in Fig. 9, PI. 29. 



PINNULARIA VIRIDIS VAR. FALLAX CL. 



Valve linear, with rounded ends; axial area narrow, slightly widened in the middle; 

 striae sometimes unilaterally interrupted, nearly parallel, 10 in 10 p. 



Elm, N. J. 



PI. 29, Fig. 4. 



In Fig. 2, PI. 30, a form is represented which corresponds closely to Navicula viridis 

 var. B, of Wm. Smith. It is given as synonymous with var. fallax; it is bilaterally inter- 

 rupted. Blue clay. 



PINNULARIA VIRIDIS VAR. ? 



Valve linear-elliptical, with rounded ends; axial area narrow, widened in the middle 

 to a transverse fascia which is sometimes unilateral; striae, 14, in the middle, divergent, 

 convergent at the ends and closer, crossed by a narrow band. L. 45-60 p.. Fascia some- 

 times absent or very narrow. 



Northbrook, Pa. 



PI. 30, Fig. 17 (represents a form with wider area than usual). 



PINNULARIA VIRIDIS VAR. CAUDATA N. VAR. 



Valve elliptical-lanceolate, with sub-rostrate ends; axial area narrow, widened to an 

 orbicular space in the middle; striae radiate in the middle, 11-12 in 10 jt, convergent and 

 closer at the ends, crossed by a narrow band; median line with very long terminal fissures; 

 terminal nodules noticeable because of the thickening of the edges of the terminal striae. 

 L. 43 M- 



Fresh water, Newtown Square. Not common. 



PI. 30, Fig. 18. 



PINNULARIA SOCIALIS (PALMER) 



Valve linear, with rounded ends; axial area broad, one-third the width of the valve; 

 striae slightly radiate in the middle, convergent at the ends, elsewhere parallel, 8 in 10 n, 

 crossed by an indistinct band about one-third the length of the striae. L. 60-120 n- 



This species, discovered by Mr. Palmer near Media, Pa., is remarkable for the group- 

 ing of the frustules "held with girdle sides together by a siliceous cementing of valve edges 

 and enclosed in a common coleodenn." The usual number included in a group is four, 

 but sometimes six or eight are noticed. The frustules adhere near their ends and are so 

 firmly fastened that boiling in nitric acid and bichromate of potash for fifteen minutes will 

 not separate them. 



Navicula socialis Palmer (Proc. Acad. Nat. Sci., Phila., 1910, p. 460, PI. 35). 



Media, Pa. 



PI. 29, Fig. 5. 



