DIATOMACE.E '. MELOSIRE^E ', COSCINODISCE.E. 



297 



slightest toucli ; and in the Infusorial earths, in which some species 

 abound, the frustules are always found detached (Fig. 145, a a, 

 d d). The meaning of the remarkable difference in the sizes and 

 forms of the frustules of the same filaments (Figs. 131, 132) has 

 not yet been fully ascertained ; but it seems to be related to the 

 curious process of self -conjugation already described (§ 219). The 



Fig 



Fig. 



Melosira subflexilis. 



Melosira variant. 



sides of the valves are often marked with radiating stria? (Fig. 

 145, d d) ; and in some species they have toothed or serrated 

 margins, by which the frustules lock-together. To this family 

 belongs the Genus Uyalodiscm, of which the H. subtilis was first 

 brought into notice by the late Prof. Bailey as a Test-object, its 

 disk being marked, like the engine-turned back of a watch, with 

 lines of exceeding delicacy, only visible by the highest magnifying 

 powers and the most careful illumination. 



228. The Family Coscinodiscece includes a large proportion of 

 the most beautiful of those discoidal Diatoms, of which the valves 

 do not present any considerable convexity, and are connected by a 

 narrow zone. The Genus Coscinodiscus, which is easily distin- 

 guished from most of the genera of this family by not having its 

 disk divided into compartments, is of great interest from the vast 

 abundance of its valves in certain fossil deposits (Fig. 144, a, a, a), 



