110 JMMTISH DESMIDIACE.K. 



chloroplast and the extremity of the cell, containing 

 one or many crystals of gypsum which exhibit a 

 constant motion. Cells in cross-section circular. 



One of the principal features of the plants of this genus is 

 the curvature of the cells. This curvature is very constant 

 for any one species and can he used as a specific character, 

 the amount of curvature of the outer margin being expressed 

 in degrees of arc. The measurement of length in this genus 

 is the straight line between the apices, and the breadth is 

 the diameter across the middle of the cell. 



Kuntze in his ' Revis. gen. plant.' substituted " Arthrodia'' 

 for Clo*terium. This name was given by Rafinesque (in 

 Desv. Journ. 1813, i, p. 235) to some Alga which, from his 

 diagnosis, might have been a Cloxterinm, Penium, Duc'ulluin, 

 Pleurotsenium, Cylindrocystis, Tetmemoru*, or even an Anki- 

 xtrudexmti*. Nordstedt (in Hedwigia, 1898, Heft 3, p. 148) 

 has pointed out that " Arthrodia'' 1 must always remain a 

 " genus igiiotum et iiomen delendum." 



In CL acntuni and a few others the chloroplasts are devoid 

 of longitudinal ridges. 



There is very little difference between Closterium and 

 certain species of Pen lam, and it is doubtful whether Penium 

 Libellula and P. uavicula should not be considered as straight 

 species of Closterium. 



At the points of division transverse lines (commonly termed 

 " sutures ;; ) are often seen, and their number denotes the 

 number of cell-divisions. In some species, in. which growth 

 takes place after cell-division, these transverse sutures occur 

 at different parts of the cell. They are useless as specific 

 characters, but the growth subsequent to division has la-en 

 utilized by Lutkemiiller as a means of subdividing the genus. 



SECTION A. Cells with a median girdle (" G-iirtelband ") or 

 cylindrical piece of cell-wall which arises subsequent to cell-division 

 and is interpolated between the new and old seniicells. 

 * Cells strongly curved, lunate ; cell- wall striated. 



1. CL Cynthia. 



2. CL Lagoe.nse. 



3. Cl. Archerianum. 



4. CL porrect/mi. 



** Cells slightly curved, ventral margin sometimes almost 



straight. 

 t Cell- wall smooth. 



5. CL didymotocuiii. 

 t>. CL macilentum. 



