o’MEARA— ON SOME NEW DIATOMACE®. 227 
WEDNESDAY, APRIL 7, 1869. 
GrorcEe Ports, Esq., M. R. I. A., in the Chair. 
Tue Minutes of the last Meeting were read and confirmed. 
Dr. A. W. Foor exhibited a fine series of Spheria Robertsii, parasitic 
on some Lepidopterous Larve from New Zealand. 
Rey. E. O’Mzara, A.M., read the following :— 
On Some New DIaTomMaceE®, COLLECTED oFF THE ARRAN IsLANDs, 
by Dr. E. Percevan Wrieut. No. III. Pl. XIII. 
Tue 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 descrip- 
tion. 
Pleurosigma giganteum, Grun., var. baccatum (Pl. XIIL., 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 Pleuro- 
sigma; on the secondary side lanceolate, with obtuse apices, nearly 
straight ; median line slightly sigmoid ; strie transverse, fine, 50—55" 
in 0:0001"; longitudinal striz somewhat finer and more remote; de- 
cussate lines extremely fine, more than 70 in 0-001”; colour in the dry 
frustule pale yellowish; length, 0:0110’—0:0170"; breadth, 0:0016” 
—0:0022”. A very well distinguished species ; I know no other that can 
be confounded with this fine Pleurosigma. The delicate structure and 
uncommon size, as well as the nearly straight figure, distinguish it from 
all others.’’* 
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 i in- 
flated in the centre; length, 0°0034”; breadth, 0:0009”; apices cu- 
neate ; vitte, two central and one at either end ; ; strize costate, pervious. 
The forms of this genus hitherto described are represented as having 
moniliform striz. Greville, in his paper on the genus Plagiogramma,t 
remarks :_—‘‘ The species are furnished in almost every instance with 
conspicuous moniliform striz. ‘There is, however, in P. ornatum an 
approach towards the coste of Odontidium.” Further on (p. 209) the 
* Grunow, ‘‘ Ueber neue oder ungenugend gekannte Algen ; ;’ “ Verhandlungen der 
kaiserlich.-kéniglichen zoologisch-botanischen Gesellschaft in Wien,” Band x., p. 559. 
+ “ Quart. Journ. Micr. Sci.,” 1859, p. 207. 
VOL. V. 2H 
