Frond simple, compressed, constricted at the middle; segments entire, with a single spine on each side.

The fronds are simple, much compressed and deeply constricted at the middle; the segments, which are broader than long, have a single spine or mucro on each side, but are otherwise smooth and entire.

The sporangia of one species only have hitherto been observed; they are spinous.

Where the plants should be placed, to the reception of which I have restricted this genus, has been left in much uncertainty. Ehrenberg, making no distinction between constricted and binate cells, has associated them with others belonging to Scenedesmus to form his Arthrodesmus; Meneghini assigns them to Staurastrum, and Kützing to Euastrum. In my papers in the ' Annals of Natural History,' I followed the arrangement of Meneghini, but at the same time suggested that these plants should be united with Arthrodesmus octocornis, Ehr., in a distinct genus, for which Ehrenberg's name might be retained. This suggestion Mr. Hassall has adopted in his ' British Freshwater Algae'; and it is approved by Brébisson. Mr. Jenner however considers that they belong to Staurastrum; and if he is correct in his supposition that he has met with one species some specimens of which were compressed, whilst others had three angles in an end view, the former arrangement ought to be restored. It must further be allowed, that in the front view the resemblance to some species of Staurastrum is very close.

Dolgelley and Penzance, J. R. Sussex and Kent, Mr. Jenner. Herts, Mr. Hassall. Yate near Bristol, Mr. Broome. Aberdeenshire, Dr. Dickie and Mr. P. Grant. Kerry, Mr. Andrews. Near Congleton, Cheshire; and Ambleside, Westmoreland, Mr. Sidebotham. Rochdale, Mr. Coates.

Germany, Ehrenberg, Kützing. Falaise, Brébisson. West Point, New York, Bailey.

Frond smooth and deeply constricted at the middle; the transversely elliptic segments have on each side a curved spine which converges with the similar one of the other segment.

The frond has a gelatinous covering, which is sometimes distinctly seen, sometimes imperceptible.

Arthrodesmus convergens differs from A. Incus by its larger size and elliptic segments.

Length of frond from 1/598 to 1/539 of an inch; breadth, not including spines, from 1/584 to 1/477; including spines, from 1/394 to 1/384. breadth at constriction 1/2368.

119

Frond smooth, minute, deeply constricted at the middle. The fronds are truncated at the ends and have a mucro at each angle; a distinct neck at the constriction is sometimes present, sometimes wanting. In alpha . the segments are semilunate and the spines usually directed outwards. The variety beta . is generally larger than alpha ; its segments are nearly equal in length and breadth, and being rather suddenly rounded off at the constriction, present a gibbouslike appearance on each side; the spines in beta. are stouter, and those of one segment slightly converge with those of the other. This form is considered a distinct species by Mr. Jenner, who thinks that its end view is sometimes triangular, and that it unites Arthrodesmus with Staurastrum.

The orbicular sporangia have been gathered in Sussex by Mr. Jenner, and I have met with them at Dolgelley and Penzance; they have subulate spines like those of Staurastrum dejectum, which they resemble, except in their smaller size.

This species is variable both in the form of its segments and in the direction of its spines; the ends however are always truncate; a character which affords a good distinction between it and A. convergens.

Euastrum Incus of Kützing's 'Phycologia Germanica' (Arthrodesmus minutus, Bréb. MS.) appears, from a drawing which he sent me, to differ merely in the smaller size and in its parallel spines. I fear it is scarcely distinct.

Arthrodesmus Incus, in the front view, has much the appearance of some states of Staurastrum dejectum; it is nevertheless usually smaller, and its ends are more decidedly truncate.

Length of frond (Tab. XX. fig. 4. a.) 1/1103 of an inch; breadth 1/1020; length of frond (fig. 4. e.) from 1/1666 to 1/1361; breadth, not including spines, from 1/1960 to 1/1420; including spines, from 1/946 to 1/793; breadth at constriction from 1/3846 to 1/3521; diameter of sporangium, without spines, 1/1210; including spines, 1/753.

Length of frond of var. beta. 1/833; breadth, not including spines, 1/1116; including spines, 1/537; breadth at constriction 1/2777.

Tab. XX. fig. 4. a, e. fronds with endochrome; b. empty frond; c, h. end views; d. frond dividing; i. fronds conjugating; k, l. sporangia; f. frond of var. beta. with endochrome; g. empty frond.