Dr. J. W. Griffitb on the Conjugation of the Diatomacese. 93 



a Navicula in a state of conjugation, and that the larger Navi- 

 cula was the sporangial frustule of the smaller, enclosed in a 

 siliceous sporangial sheath. 



The siliceous sheath (fig. 3) is colourless, elongate, rounded 

 at the ends, and furnished with coarse transverse striae or de- 

 pressions, through which the line of fracture runs when the 

 object is crushed (figs. 1 & 2). Its length its variable, appa- 

 rently according to the stage of growth. The smallest was 

 j^-Q of an inch, the parent frustules being very slightly sepa- 

 rated from each other, the form of the sheath more rounded, 

 and the transverse strise very few. The larger forms were from 

 ih to TTo" "^ length. 



The parent frustules (fig. 4, side view ; fig. 1 (below), front 

 view), of which there were two^ were brownish when dry ; linear 

 in front view ; almost linear in the side view, sometimes more or 

 less lanceolate, but always suddenly constricted near the ends, 

 which are produced. The valves have a medial line and central 

 and terminal nodules. The length of the parent frustules was 



1- to -1-" 



The sporangial frustules (fig. 5, side view ; fig. 2, front view) 

 were also brownish when diy, and in side view of a linear form, 

 slightly narrowed towards the obtuse ends ; in front view, linear 

 and narrowed towards the obtuse or slightly tnincate ends, and 

 slightly constricted in the middle; the medial line, with cen- 

 tral and terminal nodules distinct. The dimensions of the spo- 

 rangial frustules are those of the sporangial sheath. 



The detei'mination of the species I shall leave to those who 

 believe that the numerous hair-splittings of forms belonging to 

 Navicula constitute distinct species. But it is very clear that 

 the forms of the parent and sporangial frustules belong respec- 

 tively even to different sections of the genus Navicula as subdi- 

 vided by Kutzing* and Smithf. 



It would be well perhaps for those who specially study the 

 Diatomacese to attend more to the relations of the forms and 

 structure of frustules to species than to the making out of the 

 minute markings existing upon the valves, which at present re- 

 ceive almost the whole attention. The observations made in the 

 former direction are exceedingly few, yet most important to the 

 point. Those of ThwaitesJ and Focke§ have shown that great 

 differences of size and shape may exist in forms of one and the 

 same species; the subject of the present communication confirms 

 this result. Variation in the number of markings, as rays, dots, 



* Kiitzing, Species Algarum, 1849. 

 t Rev. W. Smith, Syuoj)sis of the British Diatomaceac, i. 

 + Thvvaites, Ann. Nat. Hist. 1847, xx., and pis. 4 & 22. 

 § Foeke, Physiologische Studien, 1853, Heft ii. 



