NAVICULA. 



197 



longitudinal sulcus, which on either side of the valve interrupts the costoe, 

 towards the moiety of the length. Costae robust, about 5 or 6 in 1 c.d.m. 

 Breadth, 6 to 9 c.d.m. 



Marine. Rare : Maritime Scheldt ; Blankenberghe. 



Prof. Cleve considers the two preceding forms as simply varieties of A 7 . 

 coniigua Ad. Schm., a type form which has not yet been found on our coasts. 



Valves 



furnished 



with costae. 



IV. Ellipticae. 



Valves showing 



small beads between 



the costse. 



f T , f .Sulcus approximate to 



I wo rows of u w o^it-v,;; 



,, , , raphe . . . N. Srnitnii. 



small beads ' 



between the 



1 costas. 



No 



beads 



between 



the 

 costae. 



Valves 



oval , 



rounded. 



Sulcus dividing the costas 

 into two equal parts . N. 



I A single row of beads between the costse . N. 



f Costae straight, continued . . . N. 



I 



-j Costae more or less interrupted, sometimes 

 forming thick elongated b:>ads, some- 

 (_ times simply attenuate here and there . N. 



Valve 

 moniliform, 

 not furnished - 

 with costae. 



l^Valves strictly elliptic 

 f Nodule very elongate 



. N. 

 . N. 



Nodule 



quadrangular 



or rounded. 



f Sulci abruptly flexed from above round the 



central nodule . . . . N. 



> 

 1 



I Sulci straight or flexed from below near the 

 (, central nodule ; valve broadly elliptic . N. 



nitescens 



fusca. 



suboibicularis. 



notabilis. 



advena. 



littoralis. 



elliptica. 



oculata. 



Valve appearing hyaline, with dry objectives. Striation very delicate, monili- 

 form, only becoming visible by means of homogenous objectives of wide 

 (_ aperture . . . . . . . , . . N. hya'ina. 



A. Valve furnished with coster. 

 N. Smithii Breb. (in w. Sm. S.B.D., ii., p. 92 ; H.V.H. Ad., 



pi. 9, f. 12; Supp., f. 23*; Type No. 104), plate 4, fig. 151^,/;. 



Valve oblong or linear-elliptic, sometimes somewhat constricted in the 

 middle, with apices broadly rounded, somewhat subcuneate. Central nodule 

 large, quadrangular ; terminal nodules not reaching to the apex of the valve ; 

 raphe surrounded by a rather broad hyaline zone. Sulci enlarged in the 

 middle of the valve, diminishing infinitesimally cuneate near the apices. 

 Costse feeble, 5 in 1 c.d.m., separated by two rows of small beads. Length, 

 about 9 to 10 c.d.m. 



Marine. Antwerp (Scheldt), Blankenberghe. Througnout the North Sea coasts. 



This Navicula assumes very different forms. That described above is the 

 one most frequently found at Blankenberghe and Antwerp, where it is rare. 



