Mr. J. Ralfs on the Diatomacese. 273 



the real /. ohliquata of Ehrenberg. I afterwards received from 

 Mr. 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. Areschoug, who calls it /. obliquata, is undoubtedly the 

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

 their confusion may perhaps be in great measure owing to the 

 mistake of Ehrenberg in referring the figure in ' Eng. Bot.,^ and 

 also all the habitats recorded before the publication of his work, 

 to his own /. obliquata. 



The frustules 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 series of large reticulations, in 

 shape somewhat triangular and having 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 strise ; not, 

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

 from two to four 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 VIII. fig. 2. Isthmia obliquata. 



BiDDULPHiA, Grai/. 

 Filaments attached; frustules siliceous, quadrilateral, minutely 

 reticulated, cohering by their alternate angles, and thus form- 

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

 tooth-like projections. 



Agardh founded 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 wiU prevent 

 the confusion which might othermse arise, as Ehrenberg and 

 Meyen have applied AgardVs name Odontella to a genus belong- 

 ing to the DesmidiecB. 



I have already observed under Isthmia that its structure is si- 

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

 has reticulated turgid frustules, which cohere by the elongated 

 angles. Biddulphia has also the lateral portions so inflated that 



