368 DEscEiPTioN OF [Poli/gasirica, 



F. aurea, F. (liatoiiioiihn, and F. striatula, are described ; but beiiig 

 all marine, contrary to tbe habitat of the time Fracjilaria, probably 

 belong to another genus, which Kiitzing supposes to be Grammonema 

 (Agardh.) 



Besides indicating F. virescens to be in part represented by F. pec- 

 tinalis (E), Mr. Ralfs describes (Ann. Nat. Hist., vol. xii, 1843) a 

 ep-'cies under the latter appellation, but differing in characters from 

 the similarly named one of Ehrenberg. 



'E-b.x.giulKIA. pectinalis (Ralfs.) — Frustules broad, with two evident 

 puncta at each end ; lateral surfaces striated, curved, constricted on 

 one side near the end. 



Var. {h.) — ^Found near Barmouth. F. pectinalis is brown when 

 recent, but when dried is of a pale greyish-green colour, with a 

 glass-like lustre. The endochrome is commonly contracted into two 

 irregular lines, which are not unfrequently united at the centre ; but 

 often it is in four patches, apparently from the di^^.sion of these lines. 

 The frustules sometimes have a central pellucid spot, which does not 

 appear to be connected with the endochrome. I have, several times, 

 met witli a remarkable state of this species, and have also received 

 it from Mr. Jenner. "Within the frustules, there is, apparently, 

 another siliceous frustule, the latei^i' margins of which are rounded, 

 having sti'iae like the outer frustules. In the longer frmstules it is 

 nearly elliptic ; but, in the shorter ones, appears as if truncated at 

 the ends, and, in both, it occupies the whole interior of the frustules, 

 except the corners, where the puncta at the ends are situated ; it is 

 filled with a yellowish.green granular mass, mixed with numerous 

 colourless vesicles. 



The lateral surfaces (of F. pectinalis) are very characteristic ; one 

 margin is flat, or slightly concave, the other convex, and slopes off 

 rather abruptly near the ends, where also it is slightly constricted. 

 Besides these constrictions, indications of two others may often be 

 observed on the convex margin ; but, in a specimen sent me by Mr. 

 Slaney, these are so strongly marked, that if I had not seen inter- 

 mediate forms, I should have supposed it a distinct species. I have 

 made it var. {h) — F. undulata, which perfectly agrees with the usual 

 state of the species, except in the lateral view, the convex margin of 

 which has two indentations, giving it an undulated appearance ; the 



