of the Desmidiese and Diatomese. 7 



Conjugation is far more rarely met with in the Diatomacese 

 than in the Desmidiese. It appears that this process occurs here 

 only at particular epochs,, differing according to the seasons, 

 happening simultaneously in all individuals, and quickly com- 

 pleted. Frequently as indications of conjugation having taken 

 place have been met with (the occurrence of individuals of the 

 same species, of remarkable diversity of size, side by side, in 

 free Diatomese, ex. gr. Pinnularia viridis, Surirella bifrons, Stau- 

 rosigma lacustre, all the year round ; besides the occurrence of 

 shorter or longer rows of cells of about double the diameter in 

 the bands, of the forms remaining connected by the lateral sur- 

 faces, ex. gr. Melosira, Codosira), — yet it has seldom happened 

 that they have been met with in the moment of conjugation. 

 Since the classic researches of Thwaites* upon this subject, the 

 knowledge of it has on the whole been but little advanced by 

 the observations of Focke (conjugation of Surirellai), GriffithJ 

 (conjugation oi Navicula), W. Smith § and Carter || (conjugation 

 of Cocconeis, Cymbella, Amphora). The following cases have 

 been observed : 



Formation of a single conjugation- cell, dividing very soon 

 after its origin : in Himantidium pectorale^, Cymbella Kiltzin- 

 giana^"^, Cocconeis Pediculusf-fj Cocconeis PlacentulaXX, Gom- 

 phonema lanceolatum^^, Schizonema Grevillii\\\\, Orthosira ori- 

 chalcea^^f O. Dickiei^^^, remarkable from the repeated throw- 

 ing-off of the coats of the conjugation-cell, the cracked halves of 

 which clothed the conical ends of the conjugation-cell in shape 

 of funnels; Orthosira variansfff, Surirella bifronsXXti ^^^ ^ 

 Navicula §§§, not specifically determined. Here belongs also the 

 only conjugation of a Diatomacean that I have seen, that of 



* Ann. Nat. Hist. xix. p. 200; ser. 2, i. p. 161. Summed up in Al. 

 Braun's ' Verjiingung,' p. 305 (Translation, p. 285). 



t Physiologisch. Studien, ii. p. 39. pi. 5. figs. 19-22 (1854). 



X Ann. Nat. Hist. ser. 2. xvi. p. 90. 



§ British Diatomacese, ii. p. 10. pi. a-e (1856). 



II Ann. Nat. Hist. ser. 2. xvii. pi. 1 (1856). 



II Thwaites, Ann. Nat. Hist, xix.; Smith, I. c. pi. D. f 280. i.-iii. (Smith*s 

 figures, agreeing exactly with Thwaites's, are evidently not mere copies of 

 them, but new drawings from the same preparations.) 



** Thwaites, Ann. Nat. Hist. ser. 2. i. pi. 2. figs. 1-5. Smith, pi. b. 

 fig. 47. i. ii. iii. 



ft Carter, l. c. pi. 1. fig. 2 j. XX Smith, pi. b. fig. 32. 



§§ Smith, pi. D. fig. 214. i. ||t| Smith, pi. e. fig. 364. 



11^ Thwaites, Ann. Nat. Hist. ser. 2. i. pi. 11. fig. b. (as O. crenulata) j 

 Smith, pi. E. fig. 337. 



*** Thwaites, I. c. pi. 12. figs. 1-7. 



ttt Thwaites, I c. pi. 11. fig. a. 



XXX Focke, Physiol. Studien, ii. pi. 5. figs. 19-22; pi. 6. fig. 42. (Bre- 

 men, 1854.) 



^^ J. W. Griffith, Ann. Nat. Hist. ser. 2. xvi. p. 90. 



