XXIV INTRODUCTION. 



slighter variations in each species, sufficiently constant to form 

 a safe guide to their determination, is the influence which the 

 presence of the siliceous element, in the composition of the valve, 

 has, in modifying the cellulate character of its structure, and 

 producing diversities in the number, extent, and form of its 

 markings. 



These variations of structure arise from the modes in which 

 the silica combines with the cellulose of the epiclerm ; and this 

 combination seems to obey certain and invariable laws, which 

 are subject to but slight derangement from the external circum- 

 stances in which the growth of the embryo takes place. The 

 more important variations arising from this cause exhibit them- 

 selves in the presence or absence of aggregations of the siliceous 

 material in the form of a median rib or line with its central and 

 terminal nodules, under several very characteristic modifications 

 — in the presence or absence of other transverse and strengthen- 

 ing bands of silex, and in the obvious varieties of form or com- 

 bination to which the cellules submit in the progress of their 

 formation, exhibiting themselves as hexagonal, circular, or irre- 

 gular in outline, as distinct from each other, or as more or less 

 confluent. These characters, in some cases usefid as specific 

 distinctions, are more frequently available in the arrangement of 

 the genera and their subdivisions. Less conspicuous features in 

 the cellulate structure of the valve will be found necessary in the 

 determination of specific forms. 



Whether the striae, resulting from the cellulate character of 

 the structure, are mutually parallel or radiate in their arrange- 

 ment, reach the median line, or are absent from a greater or 

 lesser portion of the valvular surface — whether the cellules 

 themselves are arranged in squares or disposed in quincunx, 

 and the striae in their transverse direction are consequently 

 parallel or oblique in reference to the margin or the median line 

 — the relative distances of the striae and their greater or less 

 distinctness, — all these are features which may safely be regarded 

 as of specific importance, and though subject to slight modifica- 

 tions, arising from the accidents of locality and age, are sufficiently 

 constant to enable us to associate together frustules which owe 

 their birth to the same sporangium, and which, on a renewal 



