40 GENEEAL HISTORY OF THE INFUSOEIA. 



Nevertheless, although a silicioiis layer be artificially separable from the 

 underlying organic coat, the relation and union of the two are indeed 

 very intimate : and in the case of the apparently inorganic external lamina, 

 the silex must be presumed to be deposited in some form of connective tissue, 

 or, in other words, to permeate it. This opinion is advocated by Meneghini, 

 who adduces in its support the circumstance of the sihcious shield of Ach~ 

 nanthidia being covered with "a very delicate dilatable membrane, itself 

 containing silica, as is proved by its sustaining imchanged the action of fire 

 and acids." This author goes on to suggest that "this permeation may 

 occur either in the wall of a simple cell, as is seen in the epidermal cells of 

 many plants, or within minute cells, as in various plants and animals." 



The surface of Diatomaceous frustules is generally very beautifully sculp- 

 tured, and the markings assume the appearance of dots (pimcta), stripes 

 (striae), ribs (costae), pinnules (pinnae) ; of furrows and fine lines ; of longi- 

 tudinal, transverse and radiating bands ; of canals (canalicuh), and of cells 

 or areolae, whilst each and all these varieties present striking modifications 

 in number, relative distribution, and in degree of development. Again, two 

 or more sorts of markings may occur together in the same individual ; and 

 lastly, the entire frustule may be covered, or certain spots may be left 

 unoccupied by them, in the form of bands, circular spaces, and the like. 



The preceding accoimt of the coverings of a Diatomaceous frustule make it 

 clear that the apparent superficial markings, although chiefly due to the 

 sculpturing of the silicious epiderm and to its internal involutions, are still 

 in some instances and in a certain degree dependent on the overlying firm 

 vegetable membrane which Mr. Shadbolt and others have shown to exist. 



But, apart from this, modem research shows that puncta, lines, costae and 

 other markings are not the same in nature in all examples presenting them ; 

 that in one case a circular point is a depression, in another an elevation, and 

 in a third a mere thickening or condensation of silicious material. So 

 of lines or costae: some are markings of the surface, and either furrows, 

 ridges or thickenings, or actual canals, whilst others are the result of invo- 

 lutions or foldings of the internal coat or incomplete septa. 



Again, the fine lines or striae of many frustules are resolvable into rows of 

 minute dots, as in Navicula and Pleurosigma. When the striae are more 

 distinctly composed of rows of dots or puncta, they are described as monili- 

 form ; examples occur in Gomplionema and Podosphenia. 



Speaking of striae, lines, and puncta generally. Prof. Smith {op. cit. i. 

 p. xvii) confesses his belief that they are all " modifications in the arrangement 

 of the silex of the valve, arising from the mode of development peculiar in each 

 case to the membrane with which the silex is combined ;" and, referiing to 

 the areolar or cellular-looking valves of Triceratium and of Isthmia especially, 

 and to the recognized growth of organized beings by cells, he arrives at the 

 conviction that " the valvular markings in every case arise from modifications 

 of cellular tissue," which forms, so to sjDeak, the matrix of the silicious 

 epiderm. " No difficulty (he adds) presents itself to the suj^position that the 

 moniliform striae of Epithemia, Navimda, and others, the circular markings of 

 Coschiodiscus eccentricus, and the iiTegular star-like stiTictiu'e of Eupodiscus 

 Argus, are aU modifications of cellular tissue ; and even in the costas of Pin- 

 nularia, and the unresolvable striae of Eupodiscus sculptus and others, it is 

 not difficult to conceive we have confluent cells whose union gives rise to the 

 appearance of lines or bands." 



Great difi'erence has existed, and even yet exists, in the interpretation of 

 the exact nature of many superficial markings. Some cuTular dots or puncta 

 are held by certain observers to be pits, by others holes, and by others to be 



