NAVICULA. 



I 9 I 



A A. Valves having either beads or irresolvable costa:. 



I. Valves with large quadrangular central nodule prolonged into horns along 



the raphe. 



a. VALVES REALLY MONILIFORM OR APPARENTLY SO IN CONSEQUENCE 

 OF THE PRESENCE OF LONGITUDINAL COST.E. 



Valves 



distinctly 



monili- 



forni. 



Valves 

 robust, 



with 

 coarse 

 beads, 



very 

 visible. 



Striation 



identical 



over all the 



valve and 



not 



interrupted 



at the 



median 



portion. 



Striation 



interrupted 



at the 



median 



portion. 



I 



f Valves small, very slightly con- 

 stricted ; sulci straight, with 

 beads not very distinct . N. didyma. 



Valves very large, conspicuously 

 constricted ; sulci dilated near 

 the central nodule, with very 

 distinct beads . . . N. fcomboides. 



Striation interrupted by a narrow- 

 band " . N. Weissflogii. 



I 



Median portion of the valve 

 hyaline, with some isolated 

 beads ; beads on the margin 

 of the valve conspicuously 

 longer than the others . N. 



[_ Valves small, with delicate beads 



N. 



Bombus. 

 vacillans. 



aa. VALVES NOT MONILIFORM, WITH COS'LE MORE OR LESS 



IRRESOLVABLE. 



Valves 



distinctly 



constricted 



at the 



median 



portion. 



Costa; showing fine transverse stria? in oblique light 



Costas not ( Striae robust, 6-8 in 1 c.d.m. 

 crossed by 



Costse 

 more or 

 less 

 irre- 

 solvable 

 into 

 coarse 

 beads. 



I 



longitudinal j Striae fine, approximate, 11-12 

 sulci. v. in 1 c.d.m. 



f A longitudinal sulcus in the 

 middle of the breadth of the 

 Costas striae .... 



crossed by I 



longitudinal \ A longitudinal sulcus following 

 sulci. the margins of the valve 



[_ Three longitudinal sulci 



Valves linear - elliptic, f" Sulcus surrounding the raphe, very broad and 



' lg the outline of the central nodule 



C Sulcus si 

 not constricted or | tracinj 

 scarcely constricted > 

 in the middle = N. 1 



contigua (A. Schm.) . Sulcus narrower, interrupted round the central 

 CI. . . . l_ nodule . 



N. constricta. 

 N. Entomon. 



N. incurvata. 



N. lineata. 



N. subcincta. 

 N. Chersonensis. 



var. Eudoxise. 

 var. Eugenia. 



II. Valves with a small central nodule, striae only present at the 



margin of the valve . . . . . . . n. musca. 



