Fragilariacece 



285 



transverse ribs or costae, between which are fine punctate striae. 

 The pseudo-raphe is indistinct. Sometimes there are slight traces 

 of longitudinal 

 This genus 



septa, 

 is distin- 

 guished from Denticula 

 by the absence of a keel 

 on the valve-faces and 

 the absence of perforated 

 longitudinal septa. 



There are four British 

 species, three of which, D. 

 vulgare Bory (length of valves 

 40 50 /i), D. elongatum Ag. 

 (length of valves up to 70 /x ; 

 fig. 131 A D) and D. hiemale 

 (Lyngb.) Heib. (fig. 131 E G) 

 are common. The two former 

 are generally distributed in 

 quiet waters, but the last- 



Fig. 131. A D, Diatoma elongatum Ag. , 

 from Wicken Fen, Cambridge. E and F, D. 

 hiemale (Lyngb.) Heib., from Howgill Fells, 

 W. Yorks. G, D. hiemale var. mesodon (Kiitz.) 

 V. Heurck, associated with the typical form. 

 (All xoOO.) 



named is more abundant in hilly districts, often occurring in pure masses 

 or mixed with a smaller variety (var. tnesodoii). 



Family 4. FRAGILARIACE^. 



In this family the cells are mostly elongate and rod-shaped. 

 They may be solitary, joined to form ribbon-like or zig-zag fila- 

 ments, or arranged in a circle like the radiating spokes of a wheel. 

 There is no keel, the valve-faces being plane or almost plane. 

 The pseudo-raphe is evident or entirely wanting, and there are 

 sometimes slight traces of central and polar nodules. The girdle- 

 view is generally rectangular, but the valve- view is lanceolate or 

 elongate, sometimes with produced apices or with one or two 

 lateral swellings. There are transverse costse or ribs and no false 

 septa, but the valve-faces are marked with transverse punctate 

 striae of variable intensity. The chromatophores are sometimes 

 small and granular, or they may be in the form of large plates. 



Genus Fragilaria Lyngb., 1819. [Odontidium Kiitz. ; Gram- 

 matonema Kiitz. ; Ralfsia O'Meara.] The valve-view is fusiform, 

 lanceolate or linear, generally with produced apices, and sometimes 

 with one or two lateral inflations. The pseudo-raphe is sometimes 

 scarcely evident, sometimes broad, and there are no nodules. The 

 valve-faces possess transverse striations, which may be exceedingly 



