102 THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 



tongue fitting into the groove. In such cases it is possible, in projection, to see the upper 

 or outer edges of the raphe, the lower edges and the edges of the tongue and groove, thus 

 showing four lines; sometimes, when the tongue and groove do not meet, six lines. The 

 so-called inner channel is the part of the raphe on the inside of the tongue, and the so- 

 called exterior channel is the part of the raphe on the outside of the tongue. If, in addition 

 to this formation of the raphe, the plane of cleavage changes toward the terminal nodules, 

 the lines will cross each other and, when two are superimposed, disappear altogether. For 

 the careful examination of the raphe it is necessary to employ large forms, and it is advis- 

 able to use nitrate of silver which remains in the raphe, and, as in slides mounted by Mr. 

 F. J. Keeley, shows in a beautiful manner the entire outline of raphe and fissures. The 

 terminal fissures owe their separation to the different directions taken by the two edges of 

 the raphe on each side, one edge bending in a wide curve toward the end of the valve, show- 

 ing two lines, the upper and lower edges of one side of the raphe when inclined to the plane 

 of the surface, and the other edge of the raphe turning suddenly in an opposite direction 

 and ending abruptly in a curve, giving rise to the appearance, by diffraction, of a punctum. 



PL 40, Figs. 13, 14 and 15. 



Endochrome consists of two chromatophores lying on the zones. 



Pinnularia is usually divided into the Majores, or larger, and the Minores, or smaller 

 forms, the latter being further divided according to their striae. The following classification 

 is chiefly that of Cleve. 



Majores. Valve large, linear with parallel or slightly radiate striae and broad axial 

 area. 



Gracillimce. Valve small, stria? parallel or nearly so; axial area very narrow. 



Capitate. Valve with capitate or rostrate ends; stria? radiate. 



Divergentes. Striae strongly radiate. 



Brevistriatce. Striae short. 



Distantes. Striae distant. 



Tabellarice. Striae radiate in the middle, strongly convergent at the ends. 



MarincE. Marine forms. 



MAJORES 

 PINNULARIA MAJOR (KUETZ.) WM. SM. 



Valve linear, usually slightly gibbous in the middle and at the ends; raphe oblique; 

 axial area less than one-third the width of valve, convergent at the ends; striae, 7 or 8 in 

 10 n, radiate in the middle, convergent at the ends, crossed by a narrow band. L. ? 

 to 300 n. 



Blue clay. Fresh water. Abundant at Middletown, Delaware Co. (T. C. Palmer). 



PI. 28, Fig. 4. 



Fig. 9, PI. 29, is one of a number of smaller forms which are difficult to determine, 

 approaching P. viridis. 



PINNULARIA MAJOR VAR. PULCHELLA N. VAR. 



Valve strongly gibbous in the middle and gradually widened to the rounded ends; 

 axial area broad, less than one-third the width of the valve, widened unilaterally in the 

 middle; striae, 7 in 10 n, crossed by a band nearly as wide as the length of the costae and 

 scarcely distinct. L. 273 p.. 



