OF THE DIATOME^. 45 



Prof. Smith likewise describes inter-lamellar channels, under the name of 

 * canalicidi/ " hollowed out between the silicious epiderm and internal cell- 

 membrane, and apparently formed by waved flexures of the epidermal enve- 

 lope They are very conspicuous in Epithemia longicornis, and form 



distinctive characters in the genera Surirella and Campylodiscus.''^ This 

 observer also regards them " as minute canals which convey the nutri- 

 mental fluid to the surface of the internal membrane," this fluid entering 

 them from without through pores or fissures existing along the line of sutiu-e 

 of the valves (p. 50). That these canals are not modifications of the cellular 

 structure of the silicious epiderm is shown by the cii'cumstance of the stri86 

 passing uninteiTuptedly over the entire surface of the valves in some Epithemice. 



The costae of Campylodiscus equally appear to be canaliculi, and are dis- 

 posed in a radiate manner. In Surirella and TryhJionella these canals are 

 usually parallel, whilst in Mastogloia they take the form of loculi. 



Kiitzing assigned a special structure and purpose to the markings he 

 called ' vittce,' and used them in forming a subsection of Diatomeae he called 

 Vittatce. The Kev. Prof. Smith remarks that to him these markings do not 

 seem special organs, but modifications in the outline of the valve, which is 

 inflected. In Grammatoplwra (XI. 48, 49, 52, 53) these inflections con- 

 stitute a leading feature of the genus, and, from their resemblance to written 

 characters, have suggested its name. In this instance they form incomplete 

 septa. 



The terms striae and costae or pinnules are not synonymous. Striae are 

 the finer lines of slight breadth, which may look like narrow grooves or 

 ridges, whilst costae or pinnules are the wider markings, having an evident 

 double contoiu', and the appearance of fissui^es or canals. The fineness of 

 some striae is such that, as before noted, they may be readily overlooked ; 

 however, their presence, when not positively demonstrable, may be assumed 

 by the coloui's displayed on focusing diied specimens. An analogous fact 

 presents itself in the case of mother-of-pearl, which owes its varying and 

 beautiful colours to the existence of fine lines covering its surface. The 

 colour varies in different species, and is due to the refraction of the rays of 

 light passing through the silicious epiderm ; its shades depend on the direction 

 of the striae and on their distance from each other ; its aid may therefore be 

 advantageously evoked in the determination of species. 



Striae generally seem to be produced by the confluence of minute rounded 

 points or beads — in other words, are commonly moniliform, and often extend, 

 as products of an investing areolar tissue, over the entire surface of the valves, 

 unlike those costae which originate in structural peculiarities of the silicious 

 plates. Rows of puncta occur in Nitzschia, and moniliform striae in Navicida, 

 Pleiirosigma, Gomphonema, and Podosplienia. To the confluence of the 

 superficial cells, Mr. Smith attributes the production of the costae of Pinnu- 

 laiia, whilst those of Achnanthes he looks upon as thickenings on the under 

 surface of the silicious valves, and generally similar to those of Istlimia, which 

 line the valves and anastomose on their under sui^face ; lastly, the striae of 

 Rhahdonema are constituted of series of oblong cells. 



The value of the external markings of Diatomeae, in a systematic point of 

 view, has been much discussed. Ehrenberg assumed the number of striae 

 or of costae or pinnules to be constant in the same space in each species, and 

 accordingly gives the number of striae counted within a given fraction of a 

 line. A great multitude of species was the consequence of this plan ; never- 

 theless the mere fact of number of striae within a given space cannot be esteemed 

 a valid specific character by itself ; for it seems quite clear that the relative 

 closeness of striae, their number within the -001 of an inch, varies according 



