NATURAL HISTORY OF THE DIATOMACEvE. 429 



those with two sets of hexagonal markings and those with only one set 

 of coarse ones, we find that under, and often filling up the whole bottom 

 of each hexagon, and therefore on the inside of the inner plate, is a little 

 plano-convex lens of silica. That such is the case is readily proved by 

 the images formed of the source of light, as a candle, by the little lenses ; 

 and in fact it would doubtless be possible to measure their focus by 

 means of a graduated fine adjustment to the microscope, such as is found 

 on the larger instruments. Sometimes the sides of the large hexagons 

 are not quite perpendicular to the inner plate, but approach each other as 

 they descend, so that the bottom of the cavity becomes concave ; and, as 

 the convexity still occurs on the inner surface of the inner plate, we have 

 a meniscus lens of silica formed, that is to say, a lens which is still 

 thickest at the centre, and therefore one which converges light like the 

 plano-convex one more commonly found. The effect on light passing 

 through such a diatom is very much the same as in the first case men- 

 tioned, but sufficiently different to be distinguished. Though these 

 peculiarities of structure can be made out by a careful observer in Aula- 

 codiscus and a few other genera, on account of their size and coarseness 

 of structure, yet they can be seen only with difficulty in others, and in 

 most diatoms cannot be shown at all. And this can be readily under- 

 stood when we remember that markings of any kind upon many species 

 can only indistinctly be seen when the best optical appliances for illumi- 

 nation and the finest microscope objectives and occulars are used. In 

 fact, it has been for the purpose of exhibiting such markings that objec- 

 tives have been specially made and apparatus invented, as has been 

 already said. 



The sculpture of the siliceous cell- wall just described is not peculiar 

 to the Aulacodiscus, but is found in many other genera. This particular 

 genus, however, is remarkable for possessing what have been called, for 

 want of a better name, "feet." These consist of tubular masses of silica 

 projecting outwards from the cell-wall, and usually placed near to the 

 margin of the valve. In some species the portion from which the feet 

 project is somewhat raised immediately under each foot, or in the form of 

 a ridge all around the valve. Within this portion the valve is either 

 plane, concave, or undulate, although the central portion is usually some- 

 what raised. The feet project outwards at a greater or less angle, and 



