192 NAVICULA. 



A . Valve furnished with robust costcc. 

 a. Two rows of small beads between the costae. 



N. Crabro Ehr. (Mikr., pi. 19, f. 29.; H.V.H., pi. 9, f. 1, 2*; 



Type No. 101), plate 3, fig. 144. 



Valve sub-elliptic, constricted or not constricted at the median portion. 

 Raphe surrounded by a narrow hyaline zone ; central nodule square, very 

 lobust. Sulci narrow, very approximate to raphe, almost straight, slightly 

 inflexed in the middle and at the apices. Costae convergent at the median 

 portion, radiate at the apices, about 3.5 to 4 in 1 cd.m., and between each 

 of which are found two rows of small beads. Length, 8 to 12 cd.m. 



Marine. Ostend (Deby). Blankenberghe (H.V.H.). England (Norm., Stolt.). Scotland, 

 Ireland (O'Me. ). Probably throughout the coasts of the North Sea. 



N. Crabro is excessively variable and all the forms are connected by 

 infinitesimal transitions ; Prof. Cleve also considers, as simple varieties, the 

 numerous forms which have been raised by authors into the rank of species. 



The forms which we give (fig. 144) consist of the var. Fandura (N. 

 Pandura Breb., N. Crabro Donk.) on the right, and the var. tnultieostata (N. 

 multicostata Grun., A T . crabro Ad. Schm.) on the left. 



a. a. A single row of beads visible between the costae. 



N. interrupta Kutz. (Bac, p. 100, pi. 29, f. 93; h.v.h. Atl., 



pl. 9, f. 7 and 8* ; Type No. 103) ; plate 3, fig. 145. 



Valve deeply constricted, with suborbicular lobes. Raphe surrounded by 

 a rather broad hyaline zone. Sulci more distant from the raphe, almost 

 straight. Costae not reaching to the margin of the valve at the constricted 

 portion, about 4.5 in 1 cd.m., straight at the median portion of each lobe, 

 convergent at the median portion of the valves, radiate at apices. A single 

 row of beads, visible with difficulty between the costae. Length, 7 to 8 

 cd.m. 



Marine Ostend (Deby). Antwerp (Scheldt). On all the shores of the North Sea. 



N. Beyrichiana Ad. Schm. (Ad. Sm. Atl., pl. 69, f. 16 and 

 17*); plate 25, fig. 728. 



Valve elliptic, with apices sub-cuneate, obtuse, constricted at the median 

 portion. Central nodule large, quadrangular. Sulci broad, showing a row of 



