INTRODUCTION. XVll 



among the most frequent appearances. Great diversity of opi- 

 nion has prevailed respecting the nature of these markings, and 

 various functions have been assigned to them, as they have been 

 regarded as apertures, protuberances, or depressions. I am, 

 hovi^ever, disposed to regard them all as modifications in the 

 arrangement of the silex of the valve, arising from the mode of 

 development peculiar in each case to the membrane with v^^hich 

 the silex is combined. It is a well-established fact, that all in- 

 crease in vegetable and animal tissues takes place by the pro- 

 duction and addition of cells ; and the law seems to prevail even 

 in the formation of the envelopes of the minute organisms with 

 which we are now concerned. Certainly no one can look at the 

 structure exhibited by the siliceous valves of Triceratium Faviis, 

 PI. V. fig. 44, and those of Isthnia enervis or /. nervosa, to be 

 given in the second volume of the present work, without feeling 

 assm'ed that he has before him the representation of a cellular 

 membrane, precisely homologous with the epidermal tissues of 

 many vegetable organs. An object-glass of sufficient power 

 and definition shows the same structure in the valves of many 

 species of Pleurodgma ; and the generalization is at once forced 

 upon the observer, that the valvular markings in every case arise 

 from modifications of cellular tissue, — a conclusion which a 

 further and closer examination does much to confirm. In no case 

 have I been able to satisfy myself of the existence of perforations 

 in the general surface of the valves ; and the existence of depres- 

 sions or elevations (except in a few cases, when such appearances 

 arise from the wavy outline of the surface,) seems to me to be 

 equally problematical; but no difficulty presents itself to the 

 supposition, that the moniliform strias of Epithemia, Navicida, 

 and others, the circular markings of Coscinodiscus eccentricus, 

 Plate III. fig. 38, and the ii-regular star-like structm-e of Eupo- 

 discus Argus, Plate IV. fig. 39, are all modifications of cellular 

 tissue ; and even in the costge of Vvnnularia and the unresolvable 

 striae of Eupodiscus scidptus, Plate IV. fig. 42, and others, it is 

 not difficult to conceive that we have confluent cells whose union 

 gives rise to the appearance of lines or bands. 



With the appearances thus arising from the cellular structure 

 of the tissue with which the silex is combined, we must not con- 



b 



