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Genus Xanthidium, Ehrenberg.
Xanthidium intermedium, sp. nov. Plate IV., fig. 33.
Frond large; constriction deep, linear; segments in front-view sub-trapezoidal, widely and roundly inflated at the base, then tapering and slightly concave towards the ends, which are wide and straight, the angles very slightly dilated. Cytioderm punctate. On each segment are a number of strong simple spines, neither dilated nor forked: most of these are disposed in groups on the edges of the basal inflations in front-view, others in pairs or in threes on the ends and at the terminal angles, a few are scattered or in short rows on the surface. Median inflation not to be made out in front-view; granules none. End-view elliptical, with a very slight median inflation; the spines arranged in a longitudinal band, most numerous towards the two ends, and with a parallel row of only a few spines at each side of the band.
Long., 89 μ; lat., 71.5 μ; crass., 35.5 μ; long. spin., 4.5 μ.
Rutherford's Swamp, Otaki.
The main distinction, it would seem, between the genera Cosmarium and Xanthidium lies in the usually truncate and granular projection which, in addition to the spines, occupies
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the centre of each segment. In the present plant this projection is so slight, so round, and so free from granules that the generic position of the plant is not easily fixed. In a general way it belongs probably to the series of X. aculeatum, Ehrenberg, but it is not that species. It is smaller than X. octonarium, Nordstedt (also a New Zealand plant), and has many more spines. Hereafter it may be found necessary to relegate it to the genus Cosmarium, but even then it will have to occupy a rather doubtful position, something like Mohammed's coffin.