for the Microscope. 33 



231. ACHNANTHIDIUM (Achnanthidium lanceolatuni). 

 The frustules resemble those of Achnanthes, but have 

 no stipes: they are united into a filament. The valves 

 are elliptical; the upper one with a median line and cen- 

 tral nodule, the lower with the nodule dilated only on one 

 side. (PI. IV., fig. 4.) 



232. BOAT-SHAPED CYMBELLA (Cymbella cuspidatd). 

 The frustules are boat- shaped, and unattached, or free. 

 The valves are striated, with indistinctly beaded or moni- 

 liform striae. (PI. IV., fig. 5.) 



233. SPEAR-SHAPED COCCONEMA (Cocconema lanceolatuni}. 

 The frustules in this genus have a similar form to those 

 of Cymbella, but are borne upon a stipes or stalk. The 

 valves have a longitudinal line near the middle, with 

 nodules at the centre and extremities. (PI. IV., fig. 6.) 



234. CLUSTERED ENCYONEMA (Encyonema caspitosuni). 

 The frustules in this genus resemble those of Cymbella, 

 but they grow enclosed in a threadlike tubular frond, often 

 much divided. The ends of the valves in this species are at- 

 tenuated. In ditches, attached to Conferva. (PI. IV., fig. 10.) 



235. MINUTE AMPHORA (Amphora minutissima). In 

 the front view elliptic or oval ; valves marked with a trans- 

 verse band. Found parasitic on other diatoms, as Nitzschia, 

 sigmoidea and Nitzschia linearis. 



236. CAPITATE GOMPHONEMA (Gomphonemacapitatuni). 

 The frustules are stalked and wedge-shaped in this 

 genus. The valves in the present species are much nar- 

 rowed towards the lower extremity. (PI. IV., fig. 7.) 



237. CONSTRICTED GOMPHONEMA (Gomphonema con- 

 strictuni). The valves differ from those of the last species 

 in being more or less constricted towards the upper ex- 

 tremity. 



238. CURVED GOMPHONEMA (Gomphonema curvatuni). 

 A small species with curved frustules, as seen in a front 

 view. The valves are narrowed gradually throughout their 

 length, with the upper extremity rounded and the lower 

 acute. 



239. LOZENGE-SHAPED NAVICULA (Navicula rhomboides) . 

 In this very large genus the form of the frustules is 



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