276 BacillariecB 



the plankton. M. Roeseana Rabenh. occurs on damp rocks among various 

 Myxophycese and also among damp mosses. The filaments of J/. granulata 

 are sometimes not more than 5p, in diameter, but those of M. arenaria reach 

 100/i in diameter (fig. 126 A and B). 



Family 2. COSCINODISCACE^. 



In this family of the Discoidese the cells are generally disc- 

 shaped, with the valves fiat, convex, or more rarely highly arched. 

 In the valve-view they are circular. There is an absence of warts 

 or other protuberances, but there is sometimes a peripheral ring of 

 spines. In some the valves possess concentric areas with different 

 types of structure, but they are not divided into sectors by special 

 radial strands. The usual type of structure consists of radial rows 

 of punctulations or areolatioiis, and there are never any excentric 

 spots. The girdle is commonly structureless or its structure is 

 almost indeterminable. The cells are solitary and free-floating. 

 The chromatophores are small and numerous, consisting of rounded 

 or lobed, parietal plates. Most of the genera of this family are 

 marine or fossil, and in many instances the distinctions between 

 them are very obscure. There are only three British genera 

 inhabiting fresh water. 



Genus Cyclotella Kiitz., 1833. The frustules are disc-shaped 

 and the valves are circular, exhibiting two concentric areas. The 

 inner area is smooth or granulate, but the outer annular area 

 possesses radiating stria?, which are smooth or punctate. Occasion- 

 ally minute spines are present near the margin of the disc. The 

 valves are excentrically bullate in most species, so that the frustule 

 seen from the girdle-view possesses undulate margins. 



There are five species known to occur in the freshwaters of the British 

 Islands. C. operculata Kiitz. (fig. 127 B and C), G. Meneghiniana Kiitz., and 

 C. Kiitzingiana Chauvin are more or less widely distributed, and C. comta 

 (Ehrenb.) Kiitz. is often abundant in the plankton. They vary in diameter 

 from 10 30/x. 



Genus Stephanodiscus Ehrenb., 1845. The valves are 

 circular, with radiating series of punctulations alternating with 

 radiating smooth spaces which present the appearance of lines. 

 In the centre of the valve there are scattered punctulations and 

 round the margin is a ring of simple acute spines. The centre of 

 the valve is generally bullate. 



