ARNOTT, ON CYCLOTELLA. 2-15 



C. Dallasiana was described by tbe late Professor Smith 

 from a single specimen; lie characterises it by the disc 

 cellular ; his slide, now in the British Museum, has been 

 carefully examined by Mr. Roper, and the valve identified by 

 him with specimens he has obtained from the Thames. 

 The disc or portion of the valve within the striated margin is 

 not areolate, as may be inferred from the term " cellular/' 

 but merely minutely bullate or puckered, or as if blistered, on 

 account of numerous little elevations and depressions ; per- 

 haps bullate-rugose is the most expressive mode of descrip- 

 tion. 



What Smith had seen is the large form, and probably the 

 sporangial state of the diatom ; but there is another form of 

 it, much smaller (with occasionally the large one sparingly 

 mixed), which I have long ago had sent me by Mr. J. T. 

 Norman, No. 178, City Road, London. This small one 

 appears to be not unfrequent about Woolwich, although it 

 has not as yet been obtained in a sufficiently pure state to 

 afford good slides. If attention be not paid to the bullate, 

 but otherwise flat, and not projecting or undulate, smooth 

 centre, this may be readily mistaken for a state of C. Kutz- 

 ingiana, Sm. ; and I have no doubt whatever that it forms 

 C. Kutzingiana ft of Smith, as far as the British localities 

 are concerned, although quite distinct from the synonyms 

 adduced. Like C. Kutzingiana, it only occurs in brackish 

 water. 



Smith states that C. Kutzingiana is met with in fresh and 

 brackish water ; I have never seen the true species from fresh 

 water, and believe that he added this kind of locality from 

 erroneously supposing that C. rectangula of De Brebisson, or 

 C. operculata ft of Kutzing, which is a fresh-water species, 

 was the same as his C. Kutzingiana ft. Specimens, however, 

 of C. rectangula, De Breb., from De Brebisson himself, and a 

 portion of the only gathering he ever made of it (near Paris), 

 prove that diatom to be no way distinct from C. Meneghiniana 

 of Kutzing, a species allied to C. Kutzingiana, and having the 

 same coarse, marginal striae; but differing by the flat, not undu- 

 late, ends, and by its fresh-water locality. 



What is called C. operculata presents two forms, both 

 figured by Kutzing. One has the centre of the valve smooth 

 and projecting obliquely (as in C. Kutzingiana) , forming, as it 

 were, a sort of operculum or convex lid to the valve, the 

 projecting portion of the one frustule corresponding to that 

 of the contiguous one which does not project, thus presenting 

 an undulate appearance on the front view ; to this the name 

 operculata properly belongs. The other has a flat (not pro- 



