234 



LIBELLUS. 



Caloneis Cleve. (rSor, in Le Diatomiste, i., p. 66). Valve with striae 

 not punctate or indistinctly punctate, and crossed by lines or longitudinal 

 bands. Prof. Cleve includes in this group all the species belonging to my 

 divisions : Abbreviated, Formosa', Limosce, and Lineares. 



Craticula Grun. (1868, Novara, p. 20). This genus was based on certain 

 anomalous valves showing transverse thickenings in the interior, such as 

 Naviada ambigua Jorma craticula. Mr. Grunow himself has now abandoned 

 this genus. 



Diadesmis Kiitz. (1844). This genus comprises certain Navicular 

 found united in long bands, such as N. gallica, Fiutowii, and contenta. 



Diploneis Ehr. (1840), created for Navicular with the valve constricted 

 in the median portion : N. didyma, Bombus, bomboides, etc. 



This genus has been revised by Prof. Cleve, who, however, bases it on the 

 peculiar form of nodule which is laterally prolonged into robust silicious 

 horns surrounding the raphe. My group of Didyma are included in the 

 genus Diploneis, with the exception of N. Musca. 



Falcatella Rabh. (1853, Suss.-Diat.) an unaccepted genus and a 

 medley of forms with arcuate valves belonging to different genera : Navicula, 

 Synedra, Nitzschia. 



Libellus Cleve. (1873, D. of Arctic Sea). Navicula? whose connective 

 membrane is plicate so as to imitate an accumulation of pages in a book. 

 The N. (Schizonemi) Grevillei and TV. Bulnheimii form part of this genus as 

 well as five other British species as follows : 



ANALYSIS OF SPECIES. 

 Valve furnished with a pseudo-stauros 



' Valve narrowly linear .... 



Valve very narrowly acute-lanceolate ; very small 



Valve 

 without 



a 

 pseudo- 

 stauros. 



Valve 



not 



narrowly 



linear. 



form 



. N. simulans. 

 . N. plicata. 



N. Bulnlieimii. 



Valve 

 rather 

 large. 



Median striae more distant than the 



others 



Striae not 



thus and 



valve only 



seen with 



difficulty in 



consequence 



of the length 



of the 

 sutural zone. 



. N. (S.) Grevillei. 



f Frustules simulating 

 an Amphora ; sutu- 

 ral zone with very 

 numerous continu- 

 ous pleats . N. complanata. 



Sutural zone very 

 broad, with very 

 numerous pleats, 

 apparently interrup- 

 ted in the middle . N. Hyalosira. 



Valve very convex, distinct ; median 

 striae not more distant ; zone with 

 numerous pleats . . N. rhombica. 



