O’Mrara—Report on the Irish Diatomacee. 281 
a subordinate feature—namely, the attachment of the frustules in fila- 
ments—has accordingly placed together such of the above-mentioned 
genera as seem to haye 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 Naviculacee. 
Grunow has adopted a group, which he has named Diatomes, 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 Fragilarieee as here defined. 
The genera which Grunow includes in the first sub-group of Diatomez 
are Odontidium, Diatoma, Plagiogramma, Fragilaria, Dimeregramma, 
his new genus Cymatosira, Grammonema, Raphoneis, Doryphora, 
Synedra, Asterionella, and Desmogonium. Of these, Grammonema is 
considered by Ralfs, Kiitzing, Ehrenberg, and Meneghini, as not 
diatomaceous. If, however, it belong to the Diatomacee, as I 
think it does, its proper position is with the Fragilaries. Dory- 
phora, Grunow describes as a stipitate Raphoneis, while in reality it is 
a stipitate Navicula, and should therefore be ranked with the 
Naviculaceze. 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 Fragilariee. Ralfs adopts Fragilaries as the designation 
of a group in which he includes Denticula, Plagiogramma, Odon- 
tidium, Fragilaria, Grammonema, Diatoma, all of which are placed by 
me in the present group; but he adds also the following very hetero- 
geneous genera:—Astcrionella, Nitzschia, Ceratoneis, and Amphi- 
pleura; while Synedra, Desmogonium, Dimeregramma, Staurosira, 
Raphoneis, under which he includes Doryphora, are ranged under 
the Surirellese. These latter genera seem to have little in common with 
the Surirellee, while, with the exception of Doryphora, already referred 
to, they exhibit the general features of the Fragilaricee ; Asterionella, 
Nitzschia, and Ceratoneis, which is unnecessarily separated. from 
Nitzschia, on account of havi ing unsymmetrical frustules, are incon- 
eruously forced into this group ; and Amphipleura, by its conspicuous 
median line, is more analogous to the N aviculaces. Lastly, Heiberg 
adopts Kiitzing’s group of Fragilarieee, which he divides into two 
sub-groups—Fragilariee genuine, and Fragilariee cuneate. 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. Fraeiarta, Lyngb.. 
Frustules on front view more or less perfectly quadrangular, 
