O'Meara — Report on the Irish Diafomacece. 281 



a subordinate feature — namely, the attachment of the frustules in fila- 

 ments — has accordingly placed together such of the ahoye-mentioned 

 genera as seem to have been known to him, with the exception of 

 Synedra, which he has ranged immediately after Pleurosigma. The 

 position thus assigned to Synedra may possibly be owing to his sup- 

 posing that the median, longitudinal, narrow, unstriated space, and 

 the unstriated central space, which some of the forms present, are 

 analogous to_ the median line and central nodule of the I^aviculaceae. 

 Grunow has adopted a group, which he has named Diatomeae, dis- 

 tributed into two sub-groups, distinguished by the absence of dia- 

 phragms in the one, and the presence of this structure in the other. 

 The former very nearly corresponds with Fragilarieas as here defined. 

 The genera which Grunow includes in the first sub-group of Diatomeae 

 are Odontidium, Diatoma, Plagiogramma, Fragilaria, Dimeregramma, 

 his new genus Cymatosira, Grammonema, Eaphoneis, Doryphora, 

 Synedra, Asterionella, and Desmogonium. Of these, Grammonema is 

 considered by Ealfs, Kiitzing, Ehrenberg, and Meneghini, as not 

 diatomaceous. If, however, it belong to the Diatomacese, as I 

 think it does, its proper position is with the Pragilariese. Dory- 

 phora, Grunow describes as a stipitate Eaphoneis, while in reality it is 

 a stipitate jS'avicula, and should therefore be ranked with the 

 !N"aviculacece. Asterionella should be excluded from this group, 

 in consequence of its unsymmetrical outline, both on the front 

 and side views ; while the general characters of Desmogonium are 

 those of the Eragilariea?. Ealfs adopts Fragilariefe as the designation 

 of a group in which he includes Denticula, Plagiogramma, Odon- 

 tidium, Eragilaria, Grammonema, Diatoma, all of which are placed by 

 me in the present group ; but he adds also the following very hetero- 

 geneous genera: — Asterionella, jSTitzschia, Ceratoneis, and Amphi- 

 pleura ; while Synedra, Desmogonium, Dimeregramma, Staurosira, 

 Eaphoneis, under which he includes Doryphora, are ranged under 

 the Surirellese. These latter genera seem to have little in common with 

 the Surirellese, while, with the exception of Doiyphora, already referred 

 to, they exhibit the general features of the Eragilarieae ; Asterionella, 

 Nitzschia, and Ceratoneis, which is unnecessarily separated from 

 Nitzschia, on account of having unsymmetrical fi'ustules, are incon- 

 gruously forced into this group ; and Amphipleura, by its conspicuous 

 median line, is more analogous to the jS^aviculaceas. Lastly, Heiberg 

 adopts Kvitzing's group of Fragilarieoe, wliich he divides into two 

 sub-groups — Eragilarieoe genuinse, and Eragilariea3 cuneatae. The 

 former, so far as it extends, corresponds with the present group, 

 while the genera contained in the latter, Meridion and Asterionella, 

 seem so incongruous that they should be placed in a widely diffe- 

 rent position. 



Genus I. Feagilaria, Lyngb. 

 Frustules on front view more or less perfectly CLuadrangiilar, 



