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tidal ; and it would, therefore, not do to lay too much stress upon the 

 native character of what was found in such a river, which was open to the 

 traffic of all the world. This would, of course, largely depend upon what 

 portion of the river the specimens came from, but if from the lower part he 

 thought such a situation to be a very likely one for the introduction of 

 foreign forms. 



Mr. Waller said that the spot where Mr. Priest had found his specimens, 

 was a part of the river beyond the reach of the tidal water. 



The thanks of the meeting were unanimously voted to Mr. Priest for his 

 communication. 



Mr. E. M. Nelson read his paper " On the Rev. Jas. Campbell's Form of 

 Fine Adjustment." 



Mr. E. M. Nelson also read a paper " On the Interpretation of Microscopic 

 Images with High Powers," illustrating the subject by diagrams. 



Mr Crisp said he quite agreed with Mr. Nelson as to the importance of 

 arriving at a proper understanding of this subject, but he thought that his 

 summary was a little more extensive than his premises justified. It was 

 interesting to see the results of Mr. Nelson's latest observations as to the 

 true nature of the markings on diatoms, and he was quite prepared to hear 

 that he had come to a clear conclusion on the matter. But whilst fully 

 recognising the great value of investigations of this kind, and not agreeing 

 with many of those cautious scientific men who often asked the question, 

 " What was the use of knowing all about it ? " he still thought that, in the 

 present state of their knowledge on the subject, Mr. Nelson was not quite 

 entitled to say that there were no such things as markings. 



Dr. Matthews thought it should be borne in mind in examining objects 

 of this kind that they were looking at something that was not fiat, and in 

 consequence of viewing them obliquely, rows of dots or perforations would 

 be apt to fall into series of lines. It was, therefore, quite possible that a 

 great deal of error in interpretation might arise from overlooking the fact 

 that they were not dealing with a flat surface. The markings were also not 

 of the same size in different parts of the same valve ; this might also tend to 

 give rise to some amount of confusion. 



Mr. Nelson said it might be interesting to state that the diagram which 

 he exhibited was drawn to scale, but it was just the reverse way to the 

 Ordnance Maps — they were on a scale of 1 in. to the mile, whereas his 

 diagrams were to a scale of 8 feet to y^ inch. 



The President said that Mr. Nelson's exposition of this subject was of the 

 highest possible interest, and he thought the details which had been given, as 

 to the minute portion of silex projecting over the round spot, formed dis- 

 tinctly a ground to work upon in carrying out these difficult inquiries. At 

 the same time he did not think he could quite agree with Mr. Nelson as to 

 the generalities of his conclusions. He had, in fact, rather a horror of 

 generalizing, for in the course of his own more particular line of investigation 

 he had found that it was a general thing for an external dermal covering to 

 be formed of three layers, and that the middle one was the supporting layer, 



