76 o'mearAj on diatomace^. 



The decision Mr. Kitton has pronounced is expressed with 

 so much doubtfuhiess, and so much that is conjectural, as 

 might reasonably, in my opinion, have suggested the propriety 

 of dealing with the subject in a gentler tone. But to give 

 colour to the verdict as it stands it is necessary to supply 

 the deficiency of facts from the suggestions of imagination. 

 It is necessary to presume that the forms are imperfectly 

 figured and described — that I am not capable of discrimi- 

 nating between a central nodule and a small grain of quartz 

 that chance has thrown in the position — that the sculpture 

 in certain portions of the valve has been obliterated by abra- 

 sion — that a certain peculiarity of structure is nothing more 

 than an abnormal marginal development. How far such 

 presumptions are warrantable, and what weight is due to a 

 judgment reached by such a process, I leave'to others to decide. 



Some of Mr. Kitton's remarks I freely acknowledge, on 

 mature consideration of them, appear not without some reason 

 to support them, though many others, as I think, afford 

 ample justification to doubt their accuracy. 



Having carefully re-examined my specimens of Navicula 

 TVriffhtii, I have no hesitation in expressing my conviction 

 that the absence of sculpture in the spaces on either side of 

 the median line is perfectly normal, not a trace of stria; is to 

 be found throughout their entire length, while on the mar- 

 ginal portion of the valve tjie stria; are in all cases perfectly 

 distinct, and exhibit no traces of the valve having been sub- 

 jected to the process of abrasion. The general resemblance, 

 indeed, between Navicular clavata, N. Hennedyi, and N. 

 Wrightii is so obvious that I consider future systematiscrs 

 would be warranted in so modifying the descriptions of these 

 forms as to include them under one denomination, but so 

 long as the two former are regarded by the authorities as 

 distinct from each other the last has a right to be regarded 

 as distinct from both. 



It is not improbable that Raphoneis Jonesii and Raphoneis 

 Moorii might be advantageously classed Avith Cocconeis 

 scutellum, to which they bear in some respects a strong family 

 resemblance, but a careful inspection of the valve, and, as I 

 think, a careful consideration of the figures and descriptions, 

 would convince that Mr. Kitton's opinion that they belong to 

 one and the same species is untenable. The sculpture in the 

 two forms exhibits a much greater diversity of structure than 

 is considered sufficient in other forms to mark diversity of 

 species. The figures, unhappily, were printed off without 

 being submitted to me for correction, but to obviate the mis- 

 take which mere inspection of the figures might lead to, I 



