CLOSTERIUM. 153 



29. Closterium sigmoideum Lagerh. & Nordst. 

 (PL XIX, figs. 1, 2.) 



Clnsterium sigmoideum Lagerh. & Nordst. in Wittr. & Nordst. Alg. 

 Exsic. 1893, no. 1138 ; Nordst. Index Desm. 1890, p. 231 ; West & G. S. 

 West, Alga-fl. Yorks. 1900, p. 53. 



Cells large, 7-8 times longer than their diameter, 

 almost straight in the front view, outer margin slightly 

 curved (about GO of arc), inner margin commonly 

 straight, gradually attenuated to the apices, which are 

 slightly recurved and obtusely rounded (rarely subtrun- 

 cate) ; when seen from the side the cells are sigmoid ; 

 cell-wall colourless and smooth ; chloroplasts with 

 8-12 pyrenoids in a somewhat irregular central 

 series ; terminal vacuoles with about twenty moving 

 granules. 



Zygospore unknown. 



Length 270-330 /x ; breadth 35-44 /x; breadth of 

 apices 6-7 /x. 



ENGLAND. Eldwick, W. Yorks ! 



Geoyr. Distribution. Ecuador. 



This is a well-marked species which we have only once 

 obtained. It occurred in abundance in a hou-^-y spring at 

 Eldwick along with the forma major. From the side (or 

 dorsal view) the cells possess a regular sio-moidal curvature 

 similar to that found normally in Pleurosigma. 



It is nearest to Cl. Lunula but is smaller and not so robust, 

 and the apices although possessing the slight but characteristic 

 recurvature of the latter species, are narrower. The chloro- 

 plasts also differ greatly from those of 67. Lunula, possessing 

 a central series of large pyrenoids. 



Forma major West & G. S. West. (PI. XIX, figs. :',-5.) 



Cl. sigmoideum forma major West & G. S. West, Alga-fl. Yorks. 1900, 

 p. 53. 



Cells larger than in the typical form, but otherwise 

 similar. 



Length 482-552 /x; breadth 47-58 /x; breadth of 

 apices 10-11 /x. 



ENGLAND. Eldwick, W. Yorks ! 



