Coscinodiscacece 



277 



St. Hantzschianus Grun. occurs in the plankton of Lough Neagh and in 

 the river Thames ; diam. of frustules 12 20/u; fig. 127 A. 



Fig. 127. A, Stephanodiscus Hantzschianus Grun. (After Schroder, x 544.) B 

 and C, Cijclotelhi operculata Kiitz., from Shipley Glen, W. Yorks. ( x 450). 

 D, Coscinodiscus lacustris Grun., from the plankton of Lough Neagh, Ireland 

 ( x 450). 



Genus Coscinodiscus Ehrenb., 1838. The valves are circular, 

 elliptical, or rarely subrhomboidal, without any striae or costse, but 

 with radiating punctulations or areolations. There is a distinct 

 edge to the disc, usually furnished with a ring of submarginal 

 spines. 



The limits of this genus are exceedingly difficult to define and "the 

 synonymy is most confusing. There are over 300 known species, mostly 

 marine, and only one occurs in the freshwaters of the British Islands. C. 

 lacustris Grun. occurs in the still waters of rivers and lakes, and reaches a 

 diameter of 60/n (fig. 127 D). It sometimes occurs in the plankton of lakes. 



Sub-order 2. SOLENOIDE^E. 



In this second sub-order of the Centricse the cells are rod- 

 shaped, many times longer than the diameter, and mostly circular 

 in cross-section. After division the cells sometimes remain 

 attached in more or less fragile chains. 



