Eunotiacece 



287 



.y. Acus (Kiitz.) Grun. are the most almndant. A variety of S. Ulna 

 var. splendens (Kiitz.) V. H. is a very striking object, the valves Ix-in.^ 

 arranged in radiating groups and reaching a length of 340 p.. <S'. capitata 

 Ehrenb. is general, but not so common as the previous species. 



Genus Asterionella Hassall, 1850. The frustules are narrow 

 and linear, with swollen 

 apices both in the valve- 

 and girdle-views ; they 

 are grouped in radiating 

 coloniesinone plane, like 

 the spokes of a wheel, 

 their basal extremities 

 being attached by deli- 

 cate mucous cushions. 

 In the valve-view the 

 cells are narrowly linear- 

 fusiform with capitate 

 apices, one apex being- 

 larger than the other. 

 In the girdle-view the 

 cells are linear with 

 swollen, truncate ex- 

 tremities. The valves 

 possess delicate transverse striations, and there is a median pseudo- 

 raphe and a hyaline area in each capitate apex. 



A. formosa Hass. (fig. 133) is a common species in the quiet waters of 

 ditches, ponds and lakes; length of valves 65 90 p.. A. granllima Heib. is 

 narrower and more elongate (length of valves up to 130 /*), and with the 

 preceding species, of which it is perhaps only a variety, is a regular and 

 abundant constituent of the British freshwater plankton. The colonies of 

 this genus are somewhat fragile and they easily become dismembered. 



Fig. 133. Asterionella formosa Hass., from 

 the plankton of Lough Neagh, Ireland ( x 450). 



Family 5. EUNOTIACE^. 



The cells are free-floating and either solitary or united by 

 their valve-faces to form ribbon-like filaments. The frustules are 

 more or less curved or arcuate in the valve-view, the dorsal border 

 of which is often undulate, the ventral margin being concave and 

 with or without a central swelling. There is either a reduced 

 raphe or a pseudo-raphe close to the ventral or concave margin, 

 and the valve is transversely striated. The central nodule is 



