INTRODUCTION. XV 



exposed to the free action of the surrounding water, without 

 secreting a sihceous epiderm ; the moment the valves become 

 separated in the process of self- division, we consequently find 

 that the secretion of a third plate of silex commences. This 

 plate forms a band between the valves, and will for convenience 

 sake be afterwards spoken of as the Connecting Membrane. 

 As self-division is continually going on while the frustules are 

 in a healthy or growing state, it is rare to find a specimen in 

 which the valves are not in some degree separated, and con- 

 sequently in which there is not more or less of a connecting 

 membrane. 



But that no such membrane exists on the first production of 

 the frustule is obvious, upon the examination of a specimen in 

 which self-division is just completed, as in PL VIII. fig. bd d; 

 PI. XVIII. fig. 167 d; and PI. XXVII. fig. 235 d, compared 

 with PI. VIII. %. 59 6 ; PI. XVIII. fig. 169 d; and PL XXVII. 

 fig. 235 6, in which the process has made some advance. The 

 process of self-division will be described hereafter ; the present 

 remarks are intended to show that the Epiderm of the Diatom, 

 strictly speaking, consists of only two parts, the connecting 

 membrane being an addition consequent upon the development 

 which attends the growth of the frustule. 



Along the line of suture in disciform or circular frustides, but 

 more generally at the extremities of the valves only, when the 

 Diatom is of an oblong, linear, or elongated form, there exist 

 perforations in the silex, which permit the surrounding water to 

 have access to the surface of the internal cell-membrane. The 

 formation of silex seems occasionally to be arrested in the neigh- 

 bourhood of these spots, and the connecting membrane is in 

 consequence either wholly or partially interrupted at such places. 

 Thus, after the internal cell-membrane is removed by acid, when 

 it often happens that the valves fall away from the connecting 

 membrane, the latter separates into two parts, and the frustule 

 has in consequence been described as consisting of four plates. 



The interruptions in the siliceous epiderm, to which reference 

 has just been made, are usually apparent as slight depressions 

 at the extremities of the frustule ; and the apjiearances they 

 present have been denominated " puncta " by Mr. Ralfs. In 



