1910.] NATURAL SCIENCES OF PHILADELPHIA. 463 



But the shallow, tabular group of Navicula socialis is in very stable 

 equilibrium upon its four raphes, and it does not readily fall a victim 

 to such accidents. 



Explanation of Plate XXXV. 



Narlcula socialis sp. nov. 



Fig. 1 — A motile group stained with Bismarck brown and tannin, showing 



the coleoderm, which invisibly encloses the group hi life, collapsed upon 



the slide under treatment. 



Fig. 2 — A motile group burned upon the cover-glass to a low red heat, leaving 



only the silicious parts, and showing contact of valve-edges at six points. 



Drawings are on a scale of 1000 diameters. 



30 



