of British species of the genus Pleurosigma. 3 



the angular aperture of the lens, and the judicious adjustment 

 of the light ; and that an instrument which may be brought by 

 the skilful manipulator to exhibit the most delicate markings of 

 these beautiful objects, may in the hands of a less familiar ob- 

 server altogether fail in the performance of its work, and appear 

 inferior to another of equal power under more experienced ma- 

 nagement. 



It must also be observed, that the employment of the high 

 powers required is liable to the intrusion of optical error, and 

 cannot be altogether depended upon. I am not indeed prepared 

 to subscribe to Schleiden's assertion, that, " if any one should af- 

 firm that he saw something magnified 3000 diameters that could 

 not be seen at a much lower power, it may safely be pronounced 

 to be mere imagination ; " yet I am persuaded that the use of 

 powers such as he alludes to can only be obtained under very 

 unfavourable conditions of light, is not available under ordinary 

 circumstances, and should not be made a requisite for the detec- 

 tion of genera or species. I have, therefore, thought it better 

 to confine myself, as far as the discrimination of species is con- 

 cerned in the figures and descriptions I am about to give, to such 

 circumstances as may be easily noted by any observer provided 

 with an instrument magnifying 400 diameters, giving however, 

 for the gratification of the more curious, a few sketches (PL I. 

 fig. 2, & PL II. figs. 17, 18, 19), drawn on a larger scale, which 

 have been kindly furnished by the Rev. W. Kingsley of Cam- 

 bridge, who has devoted great attention to this interesting sub- 

 ject, and succeeded in availing himself of powers hitherto con- 

 sidered unmanageable. 



The presence of stria; on the valves of Pleurosigma and Navi- 

 cula may be known, even when the power employed is insufficient 

 to detect lines, by the colour of the desiccated frustules viewed 

 by transmitted light. This colour differs in each species, and 

 slightly varies with the age of the individual specimen. It 

 arises from the refraction of the ravs passing through the siliceous 

 plate, and its shades depend on the direction of the striae, and 

 their distance from each other ; its aid may therefore be evoked 

 in the discrimination of species, and will sometimes be found to 

 be the most facile and certain means of identification. Another 

 means of detecting species is furnished by the direction of the 

 stria?, whether oblique, perpendicular, or parallel to the median 

 line. These circumstances are not difficult of recognition with 

 the power I have adopted, and are shown by the greater amplifi- 

 cation of Mr. Kingsley to depend upon the position of the beads 

 in reference to each other, the stria? appearing as oblique when 

 the beads of two contiguous rows are arranged alternately or in 

 quincunx, and transverse and longitudinal when these dots are 



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