Frond simple, constricted in the middle; segments compressed, entire, spinous, having a circular projection near the centre, which is usually tuberculated.

In this genus the frond is simple and deeply constricted at the middle; its segments are slightly compressed, turgid, entire, either reniform, orbicular or angular; they are furnished with spines, which, simple or branched, are either scattered or confined to the margin; in the latter case they are arranged in two rows, one on each side of the marginal line. Near the centre of each segment is a remarkable circular projection on both surfaces. These projections are usually surrounded by a beaded circle of pearly granules or tubercles, which are most evident in a lateral or transverse view.

Xanthidium requires to be distinguished from three genera, viz. Staurastrum, Arthrodesmus, and Cosmarium. Its resemblance to Staurastrum is more apparent than real, and will probably mislead no one who has examined a plant belonging to that genus, in which the cells are angular irrespective of the processes. In Arthrodesmus each segment has only two spines, one on each side, and there are no central projections.

The connection with Cosmarium is far more intimate; in fact the sole distinctive character that can be relied upon is the presence of spines in this genus. Meneghini indeed still further reduces the number and value of the differential marks, by retaining in Xanthidium merely those species whose spines are scattered over the surface, whilst he refers to Cosmarium those in which the spines are confined to the margin. I think however that his views cannot be admitted, and that we must either retain the genus as formed by Ehrenberg, or unite all the species with Cosmarium; for in X. armatum the principal spines are marginal, and many specimens, especially in a young state, have none other.

I have followed preceding writers in taking the specific characters chiefly from the form and position of the spines; but Mr. Jenner con-

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siders that the central protuberances will afford better distinctive marks of some species. However, I have seen too few examples to decide this question.

Common. North Wales; near Carmarthen; and Penzance, J. R. Sussex; Kent; Weston Bogs near Southampton, and New Forest, Mr. Jenner. Herts, Mr. Hassall. Kerry, Mr. Andrews. Ireland, Mr. Moore. Aberdeenshire, Mr. P. Grant and Dr. Dickie. Banffshire, Mr. P. Grant. Ambleside, Westmoreland, Mr. Sidebotham.

Falaise, Brébisson. Germany, Kützing.

This species forms, at the bottom of shallow pools, cloud-like masses, which being detached, immediately rise to the surface. Its fronds, dispersed like minute glittering dots through the connecting cloud-like substance, are visible to the naked eye.

Frond comparatively large, deeply constricted, the constriction forming a linear notch on each side; segments nearly equal in length and breadth, broadest at the base, their ends rounded or truncate; spines short, stout, divided at the apex; of these there are generally six pairs, which are marginal, and situated three on each side; a few others usually are scattered on the disc.

The central protuberances are cylindrical, truncate, and bordered by pearly granules which produce a dentate appearance, especially in the end and lateral views. The empty frond is minutely punctate. The end view is elliptic.

I have gathered single sporangium of Xanthidium armatum at Dolgelley; it was large and orbicular, with depressed tubercles. Having seen but this example, I am unable to say whether these tubercles eventually elongate into spines.

Xanthidium Artiscon, Ehr., differs from this species not only in its smaller size and its differently-shaped segments, but also in its longer spines, which are nearer the ends than those of X. armatum.

Length of frond 1/180 of an inch; breadth 1/270; thickness 1/280; breadth at constriction 1/625.