306 



DIATOMACE.E I — CYMBELLE^E J GOMPHONEMEJE. 



(as in Navicula), the lower valve (a) has also a transverse line, 

 forming a stauros or cross, which is wanting in the upper valve (e). 

 A persistence of the connecting membrane, so as to form an addi- 

 tional connection between the cells, may sometimes be observed 

 in this genus ; thus, in Fig. 138, it not only holds together the two 

 new frustules resulting from the subdivision of the lowest cell, a, 

 which are not yet completely separated the one from the other, 

 but it may be observed to invest the two frustules b and c, which 

 have not merely separated, but are themselves beginning to 

 undergo binary subdivision ; and it may also be perceived to invest 

 the frustule d, from which the frustule e, being the terminal one, 

 has more completely freed itself. — In the Family Cymbellece, on the 

 other hand, both valves possess the longitudinal line with a nodule 

 in the middle of its length ; but the valves have the general form 

 of those of the JZunotiece, and the line is so much nearer one 

 margin than the other, that the nodule is sometimes rather mar- 

 ginal than central, as we see in Coceonema (Fig. 145, /). — The 

 Gomphonemece, like the Meridiem and Licmophorese, have frustules 

 which are cuneate or wedge-shaped in their front view (Figs. 139, 

 140), but are distinguished from those forms by the presence of 



GompJionema gemination, more highly magnified :— a, side view of frus- 

 tule ; b, front view ; c, frustule in the act of self -division. 



the longitudinal line and central nodule. Although there are some 

 free forms in this family, the greater part of them, included in the 

 genus Oomphonema, have their frustules either affixed at their 

 bases or attached to a stipes. This Stipes seems to be formed by 

 an exudation from the frustule, which is secreted only during the 

 process of self -division : hence when this process has been com- 

 pleted, the extension of the single filament below the frustule 

 ceases ; but when it recommences, a sort of joint or articulation is 



