Royal Microscopical Society, 73 



posed clear spaces of the median band. Different valves vary- 

 considerably in minute detail. 



P. MAJOR. — The curves present less difficulties than those of 

 nohilis. The beaded ribs very delicate ; they commence in curves 

 along margin of median band, something like hooked sticks. Eows 

 of beads in median band. 



P. MESOLEPTA. — The bcst view of this elegant diatom resolves 

 the whole surface into rows of beads, median band included. In 

 form this diatom approximately resembles two skittles from a child's 

 toy box, joined at their bases. The rows of beads run obliquely 

 from median band. Crossing the median band in the centre is 

 another broad band, like two isosceles triangles with truncated 

 apices joined at their narrow ends. The spaces in these bands look 

 clear spaces with insufficient power or imperfect illumination. The 

 Hues in the triangular cross-spaces run parallel to the long diameter 

 of the valve. 



P. PEREGRiNA. — Median band and costae resolvable into rows of 

 beads. 



P. RADiosA. — Side rows of beads fall in oblique curves from 

 median band that rises like a keel. Median band exhibits rows of 

 fine beads. 



P. GiBBA. — Curves different, but structure like nohilis, &c. 



P. LATA. — This plump little diatom makes the relation of the so- 

 called costae to the median band very clear. What appear broad 

 costae under low powers, are found to be loop-like bands of beads, 

 with several rows of beads in the furrows or depressions. Median 

 band consists of rows of fine beads at right angles to costae. Very 

 troublesome to show the median dots well. 



P. FCEDERATA. — All the surfacc beaded ; beads in central nodule 

 arranged in curves. Central nodule difficult. 



P. DiVERGENS. — Median band peculiar. As seen with moderate 

 power it looks like a broad, clear space, with two narrow longitudinal 

 furrows, extending one from each end, and not meeting in the 

 middle. Each furrow expands a little at the ends nearest centre. 

 Crossing the centre are two narrow bands. With D eye-piece the 

 ^th resolves the median band into rows of beads which curve round 

 the expanded ends of the two furrows. The so-called costae re- 

 semble those of nohilis, but are less out of the horizontal plane. 



P. OBLONGA shows relation of so-called cost« to median band 

 very clearly. Symptoms of resolution in median band. 



In the preceding cases the structures described are seen with 

 various degrees of distinctness, and some under conditions more or 

 less favourable to optical illusion. It is only by comparing the 

 most distinct and clear exhibitions of the beaded structure with 

 others that are less definite that any prudent microscopist would 

 place much reliance upon the latter. 



