THE BRITISH DIATOMACEÆ.
G en u s 30. M E R ID IO N , Ag.
Frustules cuneate, united into a filament, at first attached, at length
free, and spiral ; valves elongate, convex, furnished with transverse
costæ, striated.
A close analogy exists between this genus and lAcmophora. In
both the frustules are cuneate, and adhere by their contiguous valves ;
consequently, during the process of self-division, a filament of a circular
form becomes developed. In Licmophora the circular arrangement
is speedily interrupted by the dichotomy of the stipes to which
the united frustules are permanently attached ; but in Meridian, the
attachment being confined to the earliest stages of growth, and the
frustules speedily becoming free, nothing interrupts the progress of
the circular development save the imperfect adherence of the valves,
or the occurrence of extraneous force. I t is therefore not unusual to
find the circuit completed, or the filament still farther extended into
a coil of one or two superimposed circles, though, from the fragility
of the band, it is ordinarily broken up into semicircles, or still smaller
fragments, many consisting of not more than three or four frustules.
When first developed, the single or double frustules of Meridian
bear a close resemblance to those of a Gomphonema ; but the absence
of a stipes serves to distinguish them from the stalked forms of that
genus, and the peculiar markings of the valves oifer sufficient means
of discrimination in all other cases.
As we now enter upon the filamentous species, it may be necessary