1887-88.] Remarks on the Genus Anlacodiscns Ehrb. 185 



at present known species of the genus ; in this, too, will be 

 found a guide to the relevant literature and to the synonymy 

 — here, fortunately, with few exceptions, less perplexing than 

 in some other genera ; and I would also direct attention to 

 certain singularly instructive abnormalities which have been 

 published in the April (1888) issue of the 'Journal of 

 Botany.' But by way of supplement to, and extension of, 

 these papers, it has seemed to rae desirable to present you 

 with a tabular view of the species as a guide to their natural 

 affinities, and as facilitating the identification of forms when 

 used with discrimination, in conjunction with the artificial 

 key that will be found in the first-mentioned paper. As ex- 

 planatory of this Table, I would point out that the forms 

 have a discoid shape, are sometimes almost flat or slightly con- 

 vex towards the centre, which may be depressed, and are often 

 provided with surface elevations, on the top of which, wlien 

 present, the large processes of very variable shape are placed. 

 The markings are generally absent from a small central area of 

 variable outline, but often circular, and outside of this are round 

 or polygonal areolte arranged in substraight radial and often 

 secondary oblique decussating rows, resembling the markings 

 on the back of a watch. Double rows — the primary rays — 

 of often more evident markings proceed from the central space 

 towards the border, but stop short of the latter, and for the 

 most part terminate in large and very elegant processes, which 

 vary in shape from a truncated cone to that of an hour-glass, 

 or even a round-headed staff. The markings are often of the 

 most exquisite pearly lustre, or constitute a faint delicate 

 tracery of marvellous regularity, and between them are fre- 

 quently found interspaces either of unornamented silica or bear- 

 ing minute mammillations, by the presence of which the colour 

 and lustre of the valves are often most pleasantly varied, and 

 sometimes greatly enhanced. To the forms possessing within 

 certain limits — depending on the judgment of the observer — 

 modifications of all or some of such characters the specific 

 names are attached; but it must be pointed out that here, as in 

 other departments, there is no absolute fixity of characters 

 within a species. In other words, no two valves from different 

 frustules will be found exactly the counterpart of one another, 



