O’Muara—Report on the Irish Diatomacee. 297 
ferred to the Nitzschiee ; but with Campylodiscus and Surirella, Syne- 
dra has few common characteristics. Ralfs, while he adopts this 
grouping of Kiitzing, expresses dissatisfaction with an arrangement so 
heterogeneous, and suggests that, with more propriety, Synedra should 
be ranked under the Fragilariee. It is not very easy to ascertain 
precisely what Smith’s views were as to the relations of Synedra ; for 
while in the plates the Synedre are ranged next to the Nitzschie, in 
the text they are interposed between Pleurosigma and Cocconema. 
For the reason already specified, the Synedre and Nitzschie stand 
very remote from one another, in a classification based on the sym- 
metrical or unsymmetrical structure of the frustule. Andon the same 
ground, as well as for other reasons, I cannot consider that the right 
position of Synedra is in close relation either with Cocconema or 
Pleurosigma. Grunow, either led by the suggestion of Ralfs, or by 
his own sagacity, included the genus in the first sub-group of his 
group Diatomez ; and although some genera which, for reasons spe- 
cified before, ought not to be placed in this connexion, are included 
in the sub-group, still, by this arragement, the genus was associated 
with its natural allies. Heiberg’s group of Fragilaries is nearly identi- 
cal with Grunow’s sub-group of Diatomez, the only difference being, 
that he includes in it the genus Meridion, which, in consequence of 
the unsymmetrical structure of its frustules, requires a different collo- 
cation. With the exceptions mentioned, I agree with Grunow and 
Heiberg as to the true relationship of Synedra; and in this view am 
sustained by the judgment of Rabenhorst also, who, though in his 
Siissw. Diat. he places the Synedre between the Naviculee and 
Cuneate, in his more recent work, ‘‘ Flora Europaea Algarum,”’ follows 
the more natural grouping of Grunow and Heiberg. The frustules of 
Ralfsia tabellaria, regarded separately, might be considered to belong 
to the Synedree, and were, indeed, regarded by me as identical with 
Synedra gracilis vera, not of W. Smith, but of Grunow, Verhand der 
K. K. Zool. Bot. Gesel., Band xii., 1862, p. 401, T. v., fig. 17, which 
it strongly resembles, until I had seen the frustules zm sctu, and so be- 
came convinced of my mistake. And in some cases itis difficult to 
distinguish between the separate frustules of some of the larger forms 
of Fragilaria, and some species of Synedra. I would specially refer to 
Fragilaria ungeraria, Grunow, the frustules of which, when detached, 
are scarcely, if at all, distinguishable from those of the form described 
by Kiutzing as Synedra amphirhynchus. But whatever slight con- 
fusion may arise in such cases, attention to the distinctive character- 
istics of the genus will readily remove it. 
(a.) Strie pervious ; frustules not arcuate on side view. 
Synedra chrystallina, (Lyngb.) Marine. 
Valve very long ; slightly expanded at the centre and extremities ; 
striz costate ; coste coarse; an intramarginal longitudinal line appears 
on both sides throughout the entire length. (Pl. 28, fig. 6.) 
