BRITISH DESMIDIK.K. 117 



marium and more recently an Arfhrodesmus ; whilst Meneghini and Kiitzing 

 unite it with the Micrasterice. It differs from the true species of Xanthidium, 

 not only by usually having its spines in a single series, but also by its more 

 compressed segments, and by the absence of a central projection ; for which 

 reasons Mr. Jenner proposes to make it the type of a new genus, connecting 

 as it were Xont/iidimn with Arthrodesmus or Staurastrum. Since there is 

 so much diversity of opinion, I have thought it advisable to keep it in Xan- 

 thidium until its proper situation can be determined with greater certainty. 



Length of frond ^^^ of an inch ; greatest breadth y^Vs '■> breadth at con- 

 striction 5^1^. 



Length of frond of var. /3. yoVo of an inch ; greatest breadth -gi^ ; breadth 

 at constriction -a-gVy 



Tab. XX. fig. 2. a. front view of frond with endochrome ; b. empty frond ; 

 c. end view ; d, e. fronds acquiring new segments by chvision ; f. front view 

 of a frond of var. /3. with endochrome ; g. empty frond ; h. side \'iew ; ^. end 

 view. 



10. ARTHRODESMUS, Ehr. 



Frond simple, compressed, constricted at the middle ; segments en- 

 tire, with a single spine on each side. 



The fronds are simple, much compressed and deeply constricted at 

 the middle ; the segments, which are broader than long, have a single 

 spine or mucro on each side, but are otherwise smooth and entire. 



The sporangia of one species only have hitherto been observed ; 

 they are spinous. 



Where the plants should be placed, to the reception of which I 

 have restricted this genus, has been left in much uncertainty. Ehren- 

 berg, making no distinction betw^een constricted and binate cells, has 

 associated them with others belonging to Scenedesmus to form his 

 Arthrodesmus ; Meneghini assigns them to Staurastrum, and Kiitzing 

 to Euastrum. In my papers in the 'Annals of Natural History,' I 

 followed the arrangement of Meneghini, but at the same time sug- 

 gested that these plants should be united with Arthrodesmus octo- 

 cornis, Ehr., in a distinct genus, for which Ehrenberg's name might 

 be retained. This suggestion Mr. Hassall has adopted in his ' British 

 Freshwater Alga ' ; and it is approved by Brebisson. Mr. Jenner 

 however considers that they belong to Staurastrum; and if he is 

 correct in his supposition that he has met with one species some spe- 

 cimens of which were compressed, whilst others had three angles in 

 an end view, the former arrangement ought to be restored. It must 

 further be allowed, that in the front view the resemblance to some 

 species of Staurastrum is very close. 



I 2 



