150 
On New Forms of Diatomace^e from Dredgings off the 
Arran Islands, County Galway. By the Rev. 
Eugene O’Meara. Third Series. 
(With Plate XII.) 
The forms from the Arran material referred to in this my 
third paper on the subject have already, from time to time, 
been exhibited at the meetings of the Dublin Microscopical 
Club, so that nothing remains to be done respecting them 
except to add the minute details of description. 
Pleurosigma giganteum, Grun., var. baccatum (PI. XII, 
fig. 1). — I give the general description of P. giganteum, by 
Grunow, as the best introduction to the peculiarity of this 
variety. “A very large Pleurosigma ; on the secondary side lan- 
ceolate, with obtuse apices, nearly straight; median line slightly 
sigmoid; striae transverse, fine, 50 — 55" in 0001"; longitudinal 
striae somewhat finer and more remote ; decussate lines ex- 
tremely fine, more than 70 in O'OOl"; colour in the dry 
frustule pale yellowish; length, 0'01 10" — 0'0170", breadth 
0'0016" — 0'0022". A very well distinguished species; I 
know no other that can be confounded with this fine Pleuro- 
sigma. The delicate structure and uncommon size, as well 
as the nearly straight figure, distinguish it from all others.” 
Grunow — £ Ueber neue oder ungenugend gekante Algen ; 
Verhandlungen der Kaiserlich - Koniglichen Zoologisch- 
botanischen Geselschaft in Wien,’ Band x, p. 559. 
The foregoing description of P. giganteum agrees with that 
of the form now under consideration in all respects except 
that the decussate lines referred to have not been noticed. 
The distinguishing characteristic of this variety is a chain of 
bead-like dots which adorns the outward margin as well as 
both sides of the median line. 
Plagiogramma costatum (fig. 2), n. s. — Valve sublinear, 
slightly inflated' in the centre ; length 0’0034", breadth 
0 0009"; apices cuneate ; vittae, two central and one at either 
end ; striae costate, pervious. The forms of this genus 
hitherto described are represented as having moniliform 
striae. Greville, in his paper on the genus Plagiogramma, 
‘ J. M. S.’, 1859, 207, remarks — “The species are furnished 
in almost every instance with conspicuous moniliform striae. 
There is, however, in P. ornatum an approach towards the 
costae of Odontidium.” Further on (p. 209), the striae of the 
last-named species are described as moniliform-costate. Ralfs, 
in Pritchard’s, ‘Infusoria ’ (p. 774), includes the moniliform 
