Mr. J. Ralfs on the Diatomace?e. 273 



the real I. ohliqnata of Ehrenberg. I afterwards received from 

 jNIr. Thompson the same plant gathered at the same habitat. 



The same confusion of the two species seems to prevail abroad, 

 as a specimen sent me by Mrs. Griffiths, gathered in Sweden by 

 Dr. J. E. Ai-eschoug, who calls it /. obliquata, is undoubtedly the 

 /. enervis. As the two species when compared are very chstinct, 

 their confusion may perhaps be in great measure omng to the 

 mistake of Ehrenberg in referring the figm-e in ' Eug. Bot./ and 

 also all the habitats recorded before the pviblication of his Avork, 

 to his own /. obliquata. 



The frustulcs are usually not so wide in proportion to their 

 length as in /. enervis; they are generally a little broader than 

 long, often about equal, and I have sometimes observed them 

 even rather longer than broad : in other respects their shape re- 

 sembles the preceding species. 



In the central portion, close to the lines which separate it from 

 the lateral. portions, is a single scries of large reticulations, in 

 shape somewhat triangular and haA-ing their base next to the 

 lines : in this way a beautiful beaded appearance is produced. 

 The sides have the reticulations much smaller than in /. enervis, 

 less square, and not arranged so regularly in lines. They have 

 also a few rather distant but conspicuous transverse strife ; not, 

 as in /. enervis, alternate with the reticulations, but containing 

 from two to fom- irregular series of reticulations between them : 

 this latter circumstance, combined with the more minute reticu- 

 lations and broader lines, gives this species so much more de- 

 cidedly the appearance of being striated. The strise frequently 

 anastomose. 



Plate Vlll. fig. 2. Isthtnia obliquata. 



BiDDULPHiA, Graij. 

 Filaments attached; frustulcs siliceous, quadiilateral, minutely 

 reticulated, cohering by then* alternate angles, and thus form- 

 ing a zigzag chain ; the angles are equal and elongated into 

 tooth-like projections. 



Agardh foimded his genus Odontella upon Diatoma auritum ; 

 in that plant however I am unable to find any character by which 

 it may be distinguished as a genus from Biddulphia. It is for- 

 tunate that the latter name has a prior claim, as it will prevent 

 the confusion which might othenAise arise, as Ehrenberg and 

 Meyen have appUed Agardh^s name Odontella to a genus belong- 

 ing to the Desmidiea. 



I have already observed under Isthmia that its structm-e is si- 

 milar to that of the present genus. Biddulphia, lilvc that genus, 

 has reticulated tm-gid frustulcs, which cohere by the elongated 

 angles. Biddulphia has also the lateral portions so inflated that 



