RECENT RESEARCHES IN THE DIATOMACE^. 387 



Before the cell-division there occurs a movement of the 

 enclochrome plates, which are often deeply constricted, and 

 sometimes even interrupted in the middle. Stauroneis phoeni- 

 centeron has been seen in copulation, and in this case, 

 as observed by Archer, two mother-cells produced but one 

 auxospore. 



Pleurostauruni (Rab.). — This geuus was established by 

 E-abenhorst in 1859, and thus defined — " Frustula Stauroneis 

 sed 3-5-8 in fasciara conjuncta, decussatim striata et a latere 

 visa, vittis longitudina libus fiexuosis instructa" (' Flor. 

 Europ. Alg.,' sect. 1, p. 258). To the above diagnosis Pfitzer 

 adds the peculiar unstriated border, somewhat analogous, as 

 he says, to the thickening of the walls occurring in the 

 parenchyma of the leaves of the Pine. 



In the structure of the primordial cell P. acntum and P. 

 legumen, the only species of the geims, correspond Avith 

 Navicula. A central kernel is observable. The condition 

 of the endochrome plates in the process of division has not 

 yet been ascertained. 



Pkurosigma (W. Sni.). — This genus, distinguished by the 

 sigmoid form of its valves, as respects its inner structure is 

 closely allied to Navicula. In the freshwater species P. attenu- 

 atum (Kiltz.), P. acuminatum (Kiitz. and Grunow) = P. lacus- 

 tre, W. Sm. (our author says, P. Spencei^ii, W. Sm., but this 

 is an oversight) the endochrome plates arc only notched to 

 within a little distance from the margin, just as is the case 

 with the marine species, P. fasciola, according to the descrip- 

 tion of Schultze. The larger marine forms, P. balticum 

 (Ehren.), P. angulatum (W. Sm.), P. elongatum (W. Sm.), 

 P. decorum (W. Sm.), have the endochrome plates usually 

 interrupted lay the interposition of frequent lacunae. Long 

 before the division takes place in the case of the freshwater 

 forms of Pleurosigma, the endochrome plates are cleft in the 

 middle, so that there are four of them. They then move in 

 pairs across the valves, whereupon the fission of the plasma 

 takes place. Auxosj)ores have not yet been found in the 

 genus Pleurosigma. 



Frustulia. — Supposing the peculiarities of this group to be 

 so marked as to justify the separation of the forms compre- 

 hended in it from the genus Navicula, and the establishment 

 of a new genus to receive them, the name Frustulia has not 

 been happily selected. The genus Frustulia as established 

 by Agardh embraced very heterogeneous forms ; and though 

 afterwards considerably narrowed by the author in his ' Sys- 

 tema Algarum,' remained still indefinite. The genxis was 

 adopted by Rabenhorst, and was defined (' S lissw. Diat.,' p. 50) 



