68 * NATTTEAL HISTORY SOCIETY OF DUBLIN". 



between them and such as A. bifidus (Breb.), A. octocornis, and the 

 present form, but I imagine the former are more closely related to cer- 

 tain Staurastra than are the latter to Xanthidium. For A. convergent 

 and A. octocornis may, I imagine, be almost looked on as Staurastra, two- 

 sided, not three — or more — sided, in end view — the first having most 

 affinity to such forms as Staurastrum Dickiei, or S. dejectum, or S. bre- 

 vispina, the latter having greatest relationship to S. glabrum or S. 

 G'Mearii. Three or more in number of sides (or angles) in end view, 

 in Staurastrum, is quite well known not to be of any generic or even 

 specific value, therefore two only may be of as little import. But in 

 giving expression to such a view, whatever might be the difference 

 of opinion as to generic position in the case of the species just men- 

 tioned, there cannot, I should think, be the smallest doubt as to their 

 specific distinctness ; but that is not the question at present. Arthro- 

 desmus octocornis, on the other hand, seems to possess (especially through 

 a plant I myself described as Xanthidium Smithii) considerable affinity 

 to Xanthidium, in which genus it was hesitatingly placed by Ralfs. 

 But it differs therefrom in having its marginal spines disposed in a single, 

 not a double, series, nor scattered ; and secondly, and, as I think, in a 

 more important circumstance, in its wanting the prominences occupying 

 the centre of both front surfaces of each segment characteristic of Xan- 

 thidium. Again, Arthrodesmus bifidus (Breb.) cannot at all be said to 

 be spinous, but its sub-reniform quadrangular segments are at each op- 

 posite lateral extremity simply bicuspidate, and it could not, therefore, 

 be placed in the genus Xanthidium, of which an essential and marked 

 character is to be distinctly spinous, besides the central protuberances. 

 Nor could, as I conceive, these two species (-4. octocornis and A. bifidus) 

 be placed otherwise than in the same genus, and I should be disposed 

 to take them as typical of Arthrodesmus rather than those forms whose 

 segments have a single spine only at each side. Now, notwithstanding 

 that (unlike A. octocornis) my new form has its mucrones not arranged 

 in a single series, but, as above described, possesses four at the ends of 

 the segments, as seen in end view divergent and equidistantly disposed, 

 there is yet too much generic similarity in front view, combined with 

 the absence of the central protuberances, to regard A. octocornis, my new 

 form, and A. bifidus, as belonging otherwise than to the same genus, and 

 that genus not Xanthidium, but Arthrodesmus — unless, indeed, Mr; 

 Jenner's original suggestion should be carried out, and they be made 

 into a new genus, connecting such forms as A. incus and A. convergens, &c, 

 with Staurastrum; but I -am disposed to imagine, as above indicated, 

 that the forms in question belong properly to Arthrodesmus (Ehr.), 

 whilst their allies, such as A. incus, &c, are more likely in reality tioo- 

 sided Staurastra. 



This species, like others of this family, is subject to an abnormal mode 

 of growth, by which the intervening new portions, instead of becoming 

 shut off, remain confluent, forming, with the old segments, but one unin- 

 terrupted cavity (Fig. 55). This irregularity I have myself noticed in 



