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have undergone a complete subdivision ( Cocconema, Achnanthes, 
Filabelella ). 
3rd, United in a filament or ribbon ; which is continuous, if the 
cohesion extend to the entire surfaces of the frustules ( Riabdonema, 
Fragilaria), but is sometimes a mere zig-zag chain (Diatoma, 
Grammatophora, Tabilaria), the cohesion being limited to the 
angles of the frustules. 
4th, Aggregated into a frond; which consists of numerous 
frustules, more or less regularly enclosed in a gelatinous sheath 
( Schizonema, Encyonema). These differences of condition, how- 
ever, are not made the ground of classification, because many of 
the families contain all or most of them. 
Acknowledged authorities are agreed that the group naturally 
falls into two sections, viz.—those with valves that possess a central 
longitudinal line and nodule (avicula, Pleurosigma, Pinularia, 
and Gomphonema). And secondly, including all those whose 
valves are destitute of these features (Surerella, Campylodiscus, 
Cosinodiscus ). 
The unattached forms, when the process of division has been 
completed, live a perfectly free and independent existence. They 
are most frequently met with on the surface of the mud in tidal 
harbours, roadside channels, dripping walls, and on the edges of 
grooves formed by small thread-like streams on sandy beaches. 
In calm weather, examples of the latter may frequently be met 
with on the north shore, a little this side of William Pit, where 
I have found all but one of the species discovered by Dr. Donkin 
on the shore at Morpeth. The remaining one (Zfithemia marina) 
(Donkin) I obtained once only at St. Bees. The stalked, stipitate, 
or filamentous forms, though occasionally found on stone in streams 
and pools (Meridion), are, however, usually found growing as 
parasites on sea-weed, conferva, and zoophytes. 
The sides of the piers, and the rock-pools between high and low 
water marks, furnish abundant gatherings of these varieties. The 
frondiose species are almost invariably obtained from submerged 
stones or from moss that grows under water. The last-named class 
contains species of great diversities of structure. In some, the 
