170 NAVICULA. 



N. borealis Ehr. (Verb. pi. I., II, f. 6 ; H.V.H. Atl., P . 6, f. 3*; 



Type No. 62), plate 2, fig 77. 



Valves linear-elliptic, sometimes feebly attenuated at the apices which are 

 rounded or subtruncate. Costae rather robust, distant, about 5 or 6 in t c.d.m., 

 and reaching almost to the raphe, except those in the middle, which are 

 shorter ; costge radiate at the median portion of the valve, becoming imper- 

 ceptibly convergent towards the apices. Length, 3 to 6 c.d.m. 



Freshwater. Antwerp; Frahan (Del.); probably not rare. Often found in mosses growing 

 on rocks and humid walls. Found throughout Europe. 



This type form is connected with N. lata by various intermediate 

 forms, such as N. lata Tar. minor Greg. ; N. Rabenhorstii Gru/i., etc. 

 These forms will be found in H.V.H. 's Types of Syn. No. 63, collected 

 by Mr. Delogne at Frahan. 



WITH A FALSE STAUROS. 



N. divergens W. Sm. ! (S.b.d. i., P . 57, pi. 18, f. i 7 7*inH.v.H.'s 



Types of Syn. Nos. 5 1 and 484), plate 25, fig. 706. 



Valve narrowly lanceolate, with apices somewhat attenuated. Costse robust, 

 about 5 in 1 c.d.m., not reaching to the raphe, radiate at median portion, 

 convergent towards the apices, leaving near the central nodule a broad stauro- 

 form blank space. Length of valve, 7 to 16 c.d.m. Breadth 1.5 to 2.5 

 c.d.m. 



Fresh water. Ard. Liege (De Wild.), England, Scotland, France, Gottland, etc. 



B. Costae approximate. 



a. COST.F. REACHING THE RAPHE THROUGHOUT AND SLIGHTLY SHORTENED 



NEAR THE CENTRAL NODULE. 



N. sublinearis Grun. (H.v.H. Atl, pi. 6, f. 25 and 26*) 

 plate 2, fig. 78. 



Valve narrow, linear, sometimes somewhat inflated at the median portion, 

 with rounded apices. Costa; reaching to the raphe, those round the central 

 nodule slightly shortened ; costa; fine, radiate in the middle of the valve, 

 becoming convergent by imperceptible degrees towards the apices, 21 to 24 in 

 1 c.d.m. Length, 2 to 3 c.d.m. 



Fresh water. Not yet found in Belgium. 



N. retusa Breb. (Diat. Cherb. pi. 16, f. 6. H.V.H. Atl. Supp.,f. 

 9*, in Type No. 168) plate 2, fig. 79. 



Valve linear-oblong with rounded apices. Raphe bordered by a narrow 

 hyaline zone, slightly dilated round the central nodule. Costa; robust, distant, 



