ARCHER, ON TWO NEW SPECIES OF COSMARIUM. 177 
at each opposite 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 (A. octocornis and A. bifidus) be 
placed otherwise than in the same genus, and I should be dis- 
posed to take them as typical of Arthrodesmus rather than 
those forms whose segments have a single spine only on 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 convergent 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 be- 
longing 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, &e., are more likely in reality two-sided Staurastra. 
This species, lke 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 uninterrupted cavity 
(Fig. 55). This irregularity I have myself noticed in many 
species, and it has been figured and described by several 
observers ; it is simply a monstrosity.* 
Supposing the question of the genus determined, but quite 
irrespective of it, this new form is quite distinct as a species, 
and cannot be mistaken for any other. Its size alone would 
almost distinguish it from any other Desmid ; for, notwith- 
standing that the measurements of Cosmarium tinctum, and 
of the Cosmarium I have just described above (C. pygmeum), 
are not much greater, the present species probably enjoys the 
distinction, so far as I am aware, of being the very smallest 
* See Mrs. H. Thomas, ‘ Quart. Journ. Mic. Sci.,? Vol. TI, Pl. V, figs. 
17, 18; Hofmeister, ‘Bericht der K. Sachs. Ges der Wissench. zu Leipzig,’ 
1857, Heft I; De Brébisson, ‘ Listes des Desmidiées observées en Basse- 
Normandie,” tab. i, fig. 15; De Bary, ‘ Untersuchungen iiber die Fam. d. 
Conjugaten,” p. 47, t. vi, fig. 53; W. Archer, ‘Proc. Nat. Hist. Soc. Dub- 
lin,’ vol. ii, p. 207, figs. 1O—15 ; also ‘ Nat. Hist. Review,’ O. S., vol. vi, 
p. 469, pl. xxxiii, figs. 10~--15; and again, ‘ Proc. Nat. Hist. Soc. Dublin,’ 
yol. iii, p. 37, pl. i, fig. 7; and ‘ Nat. Hist. Review,’ O. 8., vol. vii, p. 391, 
pl. xiii, fig. 7. 
