Navicula serians, N. rhomboides, and Pinnularia gibba. 163 



them, nor experiences any difficulty in restoring them to an 

 intelligible position, while, where such conjugations occur with- 

 out this nidus (as on freshwater plants, &c), they are likely, 

 especially in their advanced state, to fail in presenting some of 

 their more deciduous parts. It is perhaps on this account that 

 the ribbed sheath of the sporangia! frustule has not been more 

 frequently observed ; and as its presence, with other points here- 

 tofore unnoticed, renders a special description desirable, I shall 

 give the typical one of Navicula serians as follows : — ■ 



Navicula serians, Kg. (Smith, Synopsis, pi. 16. fig. 130). PI. IV. 

 fig. 9, enclosed in its sheath. Longest diameters 24 by 4 

 6000ths of an inch. 



Conjugation. — 1. Two frustules, varying a little more or less 

 in size, approximate themselves (PI. IV. fig. 1). 2. They secrete 

 a gelatinous substance around them, which becomes covered by 

 a delicate pellicular membrane, a a. 3. The sarcodal sacs force 

 open respectively their frustules through the fissiparating divi- 

 sional line, and carrying with them their contents, now all un- 

 distinguishably mixed together, approach each other and unite 

 into one (?) spherical mass, called the spore or sporangium, c. 

 4. The sporangium divides itself equally into two spherical 

 sporangial cells, each of which forms around itself a thick 

 opalescent capsule (fig. 2f,f). 5. The capsules respectively 

 divide in their equatorial lines, and expose the sheaths of the 

 sporangial frustules (fig. 3 g, g). 6. The sheaths become elon- 

 gated, and at the same time present thread-like rings on their 

 surface, diminishing gradually in thickness towards each ex- 

 tremity, but in close approximation throughout, except at the 

 middle, where there is an interval of ti \ th of an inch between 

 them, or equal to three times the breadth of the neighbouring 

 rings (figs. 4, 6, 7 h, i). 7. The sheaths, elongating, carry out 

 the ends of the capsules upon their extremities (figs. 3-8/). 

 8. The hemispheres of the capsules disintegrate into short, fusi- 

 form, curved, thread-like filaments (fig. 8/). 9. The sheath 

 is fully formed, and the sporangial frustule appears within it 

 (fig. 9). 10. The empty sheath presents a longitudinal dehis- 

 cent fissure, through which the sporangial frustule obtains its 

 exit (fig. 10). 



Having only observed one instance in which the longitudinal 

 fissure of the sheath was present, I am not quite certain that 

 the frustule always obtains its exit through this kind of dehis- 

 cence; nor am I certain that the frustule always comes forth 

 singly, having often seen it in duplication within the sheath, by 

 which operation it might be the better able to force open the 

 latter. 



11* 



