MICROSCOPIC FORMS OF VEGETABLE LIFE. 



289 



which appear in the front view as in Biddulphiece ( 292). The typical 

 form of this family is the Terpsinoe musica, so named from the resem- 

 blance which the markings of its costae bear to musical notes. 



288. We next come to two Families in which the lateral surfaces of 

 the frustules are circular ; so that, according to the flatness or convexity 

 of the valves and the breadth of the intervening hooped band, the frus- 

 tules may have the form either of thin disks, short cylinders, bi-convex 

 lenses, oblate spheroids, or even of spheres. Looking at the structure 

 of the individual frustules, the line of demarcation between these two 

 families, MelosirecB and Coscinodiscece, is by no means distinct; the prin- 

 cipal difference' between them being that the valves of the latter are com- 

 monly cellulated, whilst those of the former are smooth. Another 

 important difference, however lies in this, that the frustules of the Cos- 

 cinodiscece are always free, whilst those of the Melosirece remain cohe- 

 rent into filaments, which often so strongly resemble those of the simple 

 ConfervacecB as to be readily distinguishable only by the effect of heat. Of 

 these last the most important Genus is Melosira (Figs. 177, 178). Some 

 of its species are marine, others fresh-water; one of the latter, the M. 

 ochracea, seems to grow best in boggy pools containing a ferruginous 

 impregnation; and it is stated by Prof. Ehrenberg to take up from the 

 water, and to incorporate with its own substance, a considerable quan- 

 tity of iron. The filaments of Melosira very commonly fall apart at the 

 slightest touch: and in the Infusorial earths, in which some species 

 abound, the frustules are always found detached (Fig. 192, a a, d d). 



The meaning of the remarkable 

 difference in the sizes and forms of 

 the frustules of the same filaments 

 (Figs. 177, 178) has not yet been 

 fully ascertained ( 281). The 

 sides of the valves are often mark- 

 ed with radiating striae (Figs. 192, 

 d d); and in some species they 

 have toothed or serrated margins, 

 by which the frustules lock- to- 

 gether. To this family belongs 

 the genus Hyalodiscus, of which 

 H. suUilis 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 ex- 

 ceeding delicacy, only visible by 

 the highest magnifying powers 

 and the most careful illumina- 

 tion. 



Melosira subflexilis. Melosira varians. 289. The family CoSCinodlSCeCB 



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 distinguished 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 fos- 

 sil deposits (Fig. 191, a, a, a), especially the Infusorial earth of Eich- 

 mond in Virginia, of Bermuda, and of Oran, as also in Guano. Each 

 19 



