608 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



frustules, which appear in the front view as in Biddulphiece. The 

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

 the resemblance which the markings of its costse bear to musical 

 notes. 



We next come to two families in which the lateral surfaces of 

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

 vexity of the valves and the breadth of the intervening hooped band, 

 the frustules may have the form either of thin discs, short cylinders, 

 biconvex lenses, oblate spheroids, or even of spheres. Looking at 

 the structure of the individual frustules, the line of demarcation 

 between these two families, Melosirece and Coscinodiscece, is by no 

 means distinct, the principal difference between them being that 

 the valves of the latter are commonly areolated, whilst those of the 

 former are smooth. Another important difference, however, lies in 

 this, that the frustules of the Coscinodiscece are always free, whilst 

 those of the Melosirece remain coherent into filaments, which often 

 so strongly resemble those of the simple Confervacece as to be readily 

 distinguishable only by the effect of heat. Of these last the most 

 important genus is Melosira (fig. 444). Some of its species are 

 marine, others fresh-water ; one of the latter, M. ochracea, seems to 

 grow best in boggy pools containing a ferruginous impregnation ; 

 and it is stated by Professor Ehrenberg that it takes up from the 

 water, and incorporates with its own substance, a considerable 

 quantity 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. 468, a, a, d, d). The meaning of the remarkable difference in 

 the sizes and forms of the frustules of the same filaments (fig. 444) 

 has not yet been fully ascertained. The sides of the valves are 

 often marked with radiating striae (fig. 468, d, d)~ and in some 

 species they have toothed or serrated margins, by which the frustules 

 lock together. To this family belongs the genus Hyalodiscus, of 

 which H. subtilis was first brought into notice by the late Professor 

 Bailey as a test-object, its disc being marked, like the engine-turned 

 back of a watch, with lines of exceeding delicacy, only visible by 

 good objectives and careful illumination. 



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 distinguished from 

 most of the genera of this family by not having its disc divided into 

 compartments, is of great interest from the vast abundance of its 

 valves in certain fossil deposits (fig. 467, a, a, a) especially, the 

 infusorial earth of Richmond in Virginia, of Bermuda, and of Oran, 

 as also in guano. Each frustule is of discoidal shape, being com- 

 posed of two delicately undulating valves united by a hoop ; so that 

 if the frustules remain in adhesion, they would form a filament 

 resembling that of Melosira (fig. 444, B). The regularity of the 

 hexagonal areolation shown by its valves renders them beautiful 

 microscopic objects ; in some species the areolse are smallest near 

 the centre, and gradually increase in size towards the margin ; in 



