O'Meara — Re2)ort on the Irish Diatomacece. 297 



ferred to the Mtzschiese ; but with Campylodiscus and Surirella, Syne- 

 dra has few common characteristics. E-alfs, while he adopts this 

 grouping of Kiitzing, expresses dissatisfaction with an arrangement so 

 heterogeneous, and suggests that, with more propriety, Synedra should 

 he ranked under the Pragilarieae. 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 Synedrae are ranged next to the Mtzschige, in 

 the text they are interposed between Pleurosigma and Cocconema. 

 Por the reason already specified, the Synedrae and ISTitzschiae stand 

 very remote from one another, in a classification based on the sym- 

 metrical or unsjnoimetrical striicture of the frustule. And on 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 Ealfs, or by 

 his own sagacity, included the genus in the first sub-group of his 

 group Diatomeae ; 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 Pragilarieae is nearly identi- 

 cal with Grrunow's sub-group of Diatomeae, 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 Grrunow and 

 Heiberg as to the true relationship of Synedra ; and in this view am 

 sustained by the judgment of E.abenhorst also, who, though in his 

 Siissw. Diat. he places the Synedrae between the ISTaviculeae and 

 Cuneatae, in his more recent work, "Mora Europaea Algarum," follows 

 the more natural grouping of Grrunow and Heiberg. The frustules of 

 Ealfsia tabellaria, regarded separately, might be considered to belong 

 to the Synedrae, and were, indeed, regarded by me as identical with 

 Synedra gracilis vera, not of W. Smith, but of Grrunow, Yerhand 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 in situ, and so be- 

 came convinced of my mistake. And in some cases it is difficult to 

 distinguish between the separate frustules of some of the larger forms 

 of Pragilaria, and some species of Synedra. I would specially refer to 

 Tragilaria ungeraria, Grrunow, the frustules of which, when detached, 

 are scarcely, if at all, distinguishable from those of the form described 

 by Kiitzing 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.) Stri(B pervious ; frustules not arcuate on side view. 



Synedra chrystalUna, (Lyngb.) Marine. 



Yalve very long ; slightly expanded at the centre and extremities ; 

 striae costate ; costae coarse ; an intramarginal longitudinal line appears 

 on both sides throughout the entire length. (PI. 28, fig. 6.) 



