Mr. J. Ralfs on the British Desmidiese. 149 



XXI. — On the British Desmidiese. By John Ralfs, Esq., 

 M.R.C.S., Penzance*. 



[With a Plate.] 

 Staurastrum, Meyen. 



Fronds simple, constricted in the middle ; end view angular, or 

 circular with the margin lobato-radiate, or in a few instances 

 compressed with a process or mucro at each extremity. 



Fronds minute, simple, more or less constricted in the middle, 

 so as to form two segments, which are often somewhat twisted, 

 generally broader than long, and in most of the species elongated 

 laterally into a process, so that the constriction on each side is a 

 roundish or angular sinus ; in other respects the front view shows 

 the segments quite entire. 



The end view varies in form ; in most of the species it is tri- 

 angular or quadrangular, and the angles are either rounded or 

 elongated into rays ; in a few it is circular, with five or more pro- 

 cesses forming marginal rays; and in three species it is compressed, 

 and the extremities terminate in either a process or a mucro. 



Ehrenberg in his great work has distributed the plants which 

 I shall describe here among different genera, according to the 

 number of angles or processes seen in an end view. Thus he 

 refers those with three angles to Desmidium, and those with four 

 to Staurastrum. He formed his genus Pentasterias for the recep- 

 tion of a plant with five rays, and placed one with two processes 

 in his new genus Arthrodesmus. But this arrangement appears 

 unnatural, not only because it separates nearly allied forms, but 

 also because the number of rays are not constant even in the 

 same species, as Meneghini remarks; whilst Professor Bailey says, 

 when describing an American species, " The number of arms is 

 usually three, but 1 have met with specimens in which one cor- 

 puscle had three and the other four arms, others in which both 

 had/owr, and others again in which both had /ve arms;^^ I have 

 myself seen a frond of Staurastrum paradoxumy one segment of 

 which had four and the other only three rays. I have generally 

 found the Pentasterias margaritifera of Ehrenberg having six rays, 

 but not unfrequently five, and occasionally indeed even seven rays 

 to each segment. 



In the following description of this genus I have taken as my 

 guide Meneghini^s ' Synopsis Desmidiearum,^ the best work on 

 this family which I have seen. Besides the species placed in this 

 genus by Ehrenberg, I have included in it his Pentasterias, and 

 those plants which he has improperly united with Desmidium, and 



* Read before the Botanical Society of Edinburgh, May 9, 1844. 



