RYLANDS, ON MARKINGS OF DTATOMACE.E. 



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We learn, by a subsequent communication from H. M., 

 that he has tried other arcs, as 15° and 10°, by the above 

 test, and finds that the error of the Formulae of Approxima- 

 tion in these cases exceeds that which is here proved to exist 

 in the arc of 40°. 



He has also found that Mr. Potter's third approximation is 

 incorrect as well as the second, which is given in the Example. 



On the Markings o/Diatomace.e. 

 By Thos. G. Rylands, Esq. 



It is not easy to over-estimate the importance of the 

 department " Notes and Correspondence" in your Journal. 

 Not only does it afford a means of recording facts which 

 might otherwise remain unpublished ; but the various hints 

 and memoranda contained in it possess a value to the country 

 student which those only who are similarly situated can 

 appreciate. The note by Dr. Greville, " On a Structure 

 observed in Surirella/ vol. vii, p. 116, recalled my attention 

 to the subject of the present article, and reminded me that 

 on two occasions I had observed something similar in 

 naviculoid frustules. The first was in Pleurosigma halticum, 

 where the internal marking was hexagonal j and the second 

 in a frustule of Amphipleura pellucida, where there were 

 transverse lines, but much more distant than the 130 to 

 "001" observed in that species by the Hull microscopists. 



