4 The Rev. W. Smith on the Diatomacese. 



impression of the numerous and prominent valvular markings of 

 this beautiful species. 



But to proceed with the process of self-division. At first the 

 siliceous valves are in close contact at their suture, as may 

 be seen in PI. II. fig. 1 h, and PI. III. fig. 3 c, but their adhei'ence 

 is speedily disturbed by the dividing process which these minute 

 organisms are constantly undergoing. The first step in this 

 process is the gradual separation of the valves, an effect ap- 

 parently produced by the expansion of the internal membrane. 

 Pari passu with the retrocession of the valves, the cell- wall 

 exposed between their edges is being covered with a deposit of 

 silex, and the frustule now consists of two symmetrical valves, 

 united by a plate of silex (PI. II. fig. 1 o), which either forms a 

 continuous ring (PL II. fig. 1/) or consists of two portions united 

 at the extremities of the valves. This plate with the underlying 

 cell-wall, may for the sake of distinctness and future reference 

 be termed the connecting membrane. 



When the connecting membrane has been formed of sufficient 

 width, the original cell, probably by the doubling in of its wall, 

 becomes divided into two, and immediately seci'etes, at the line 

 of division, two new siliceous valves, symmetrical with and closely 

 applied by theii* edges to the original halves, and thus the self- 

 division is complete, and two perfect frustules have been the 

 result (PI. II. fig. 1 h, & PI. III. fig. 2 c). 



In some cases, by the new or rather semi-new frustules imme- 

 diately proceeding to repeat the process, the connecting mem- 

 brane is thrown ofi" and disappears ; in others it remains for some 

 time linking the frustules in pairs, as in Melosira and Odontella ; 

 and sometimes it is only partially torn away or absorbed, and 

 unites the frustules successively formed in a zigzag chain by por- 

 tions remaining attached to their angles, examples of which we 

 find in Diatoma and Isthmia, &c. 



Late wi'iters have found in the process of self-division circum- 

 stances to fix the terminology applied to the Diatomaceous frus- 

 tule, and use the words " primaiy sides " when speaking of 

 those portions where the interposition of the new half-frustules 

 occui's ; the term " secondaiy sides " being applied to the ge- 

 neral surfaces of the valves : others employ the words " front " 

 and " lateral view " in corresponding senses. I shall adopt the 

 latter tenns, as more generally applicable ; the " primaiy side," 

 as employed by the writers alluded to, frequently including 

 portions of the frustule which belong to the secondaiy sm*- 

 faces, brought into view by the convexity of the valves. In 

 truth, it is difiicult to fix upon terms always applicable to 

 foi-ms so varied ; T tnist however ambiguity will be avoided by 

 my adopting in the following descriptions the language hitherto 



