DIATOMACE/E '. FAMILY FKAGLLLARIEJE. 293 



Tlie newly-formed part of the stipes in the Genus Licmophora, 

 instead of itself becoming double 'with each act of self-division of 

 the frustule, increases in breadth, while the frustules themselves 

 remain coherent ; so that a beautiful fan-like arrangement is pro- 

 duced (Fig 126). A splitting away of a few frustules seems occa- 

 sionally to take place, from one side or the other, before the 

 elongation of the stipes ; so that the entire plant presents us with 

 a more or less complete flabella or fan upon the summit of the 

 branches, with imperfect flabellse or single frustules irregularly 

 scattered throughout the entire length of the footstalk. This 

 beautiful plant is marine, and is parasitic upon Sea-weeds and 

 Zoophytes. 



224. In the next Family, that of Fragillarieo?, the Frustules 

 are of the same breadth at each end, so that if they unite into a 

 filament they form a straight band. In some genera they are 

 smooth, in others transversely striated, with a central nodule ; 

 when' stria? are present, they run across the valves without inter- 

 ruption. To this family belongs the Genus Diatoma, which gives 

 its name to the entire group : that name (which means cutting 

 through) being suggested by the curious habit of the genus, in 

 which the frustules after self-division separate from each other 

 along their lines of junction, but remain connected at their angles, 

 so as to form zig-zag chains (Fig. 127;. The valves of Diatoma, 

 when turned sideways (a), are seen to be strongly marked by 

 transverse striae, which extend into the front view. The propor- 

 tion between the length and the breadth of each valve is found to 

 vary so considerably, that, if the extreme forms only were com- 

 pared, there would seem adequate ground for regarding them as 

 belonging to different species. This genus inhabits fresh water, 

 preferring gently-running streams, in which it is sometimes very 

 abundant. The Genus Fragillaria is nearly allied to Diatoma, 

 the difference between them consisting chiefly in the mode of 

 adhesion of the frustules, which in Fragillaria form long straight 

 filaments with parallel sides ; the filaments, however, as the name 

 of the genus implies, very readily break-up into their component 

 frustules, often separating at the slightest touch. Its various 

 species are very common in pools and ditches. This family is 

 connected with the next by the Genus Nitzsckia, which is a some- 

 what aberrant form distinguished by the presence of a prominent 

 keel on each valve, dividing it into two portions which are usually 

 unequal, while the entire valve is sometimes curved, as in N. sig- 

 moidea, which is sometimes used as a Test-object, but which is 

 not suitable for that purpose on account of the extreme variability 

 of its striation, Nearly allied to this is the genus Bacillaria, so 

 named from the elongated staff-like form of its frustules ; its valves 

 have a longitudinal punctated keel, and their transverse striae are 

 interrupted in the median line. The principal species of this 

 genus is the B. paradoxa, whose remarkable movement has been 



