26 RYLANDS, ON MARKINGS 01' Dl ATOM AClvE. 



In neither of these instances were the slides my own, and 

 they were not seen under circumstances which enabled me 

 to do more than record the fact of their occurrence. In 

 neither case was the idea of a septum entertained; the 

 internal markings had distinctly the curve of the side of 

 the frustule, and the impression conveyed was simply that 

 both surfaces of the silica were marked. I had the less 

 hesitation in coming to this conclusion, from having fre- 

 quently seen something similar, but apparently constant, in 

 certain disciform species. The fact that certain species of 

 Actinocyclus, &c, have two very evident sets of markings is 

 generally known. It is hardly less probable that the same 

 thing has been seen in Coscinodiscus centralis, and that the 

 fine secondary markings of Triceratium J'avus (indicated by 

 Dr. Wallich in what he calls T. fimbriatum) have been 

 observed by those who have directed their attention to that 

 species.* 



It was while I was looking for the same thing in Coscino- 

 discus radiatus, that the importance of separating and dis- 

 tinguishing the several types of markings was impressed upon 

 my mind. Until they are so distinguished, their real value as 

 characters for purposes of classification must remain un- 

 known. How far I have succeeded in this investigation 

 remains to be proved, and Avhen our knowledge will be suf- 

 ficient for practical purposes depends upon the attention 

 which observers may hereafter give to the subject. I may 

 mention that so long as observation is confined to perfect 

 specimens little progress will be made ; fragments now and 

 then occur happily placed at the right angle for the purpose, 

 and convey more information at a glance than can be 

 obtained in hours from the ordinary side or front view of a 

 i'rustule. 



With the hope that I may hereafter be in a position to 

 enter more fully into this subject, I shall refer now only to 

 those markings which bear upon the purpose for which this 

 article is written. 



The siliceous portion of diatomaceous frustules seems 

 to be normally composed of two layers, more or less 

 intimately combined. The connexion may be simple and 

 entire, as in the case of Pleurosigma angvlahm. Indi- 

 vidual specimens of this species are far from uncommon, in 

 which the outer " areolated" layer is partially removed, 

 Leaving the inner layer entire. Isolated portions of the 



* 'Micr. Journ.,' vol. vi, p. 21?, pi. xii, figs. 4—9. 1 need hardly add 

 thai the marginal fimbriae and the minute processes at the angles of the 

 hexagonal arc;c also occur in British specimens of T.favus. 



