64 BRITISH DESMIDIE.E. 



on the contrary, the joints have two prominent teeth at the margins 

 or angles, and the foramen results from the excavation or concavity 

 of the joint itself, and not from the presence of glands. 



1. A. Desmidium (Ehr.) ; joints in the front view quadrangular, 



broader than long ; crenatures distinct. 



a. Filaments triangular, regularly twisted ; crenatures rounded. 



Desmidium aptogonum, Breb. Alg. Fal. p. 65. t. 2. (1835). Menegh. Syn. 

 Desm. in Linn. 1840, p. 203. Kiitz. Phy. Germ. p. 141. 



/3. Filaments plane; crenatures shallower and slightly angular. 



Odontella Desmidium, Ehr. In/us. p. 153. t. 16. f. 4. (1838). 



a. Ambleside, Westmoreland, Mr. Sidebotham. Dolgelley, J. B. 



/3. Ambleside, Mr. Sidebotham. 



North Germany, Ehrenberg. Carlsbad, Corda. Falaise, Brebisson. 



The filaments, which are slenderer than those of Desmidium Swartsii, are 

 slightly mucous, but have no distinct sheath. The joints are nearly as long 

 as broad, and being excavated at their junction and connected only at the 

 margins, they leave oval foramina between them. 



The two states of this plant are so different that they seem at first sight 

 distinct species. In a. the filaments are triangular, and being regularly twisted 

 as in Desmidium, they in like manner exhibit a dark waved longitudinal line, 

 marking the third angle ; hence the foramina are far less conspicuous in this 

 than in the plane variety, as they are obscured by the junctions of the third 

 angle. When the dark line which denotes that angle approaches the margin 

 of the filament they are more evident, and their size and figure may be ascer- 

 tained. The marginal crenatures are deeper and more romided, which fact, 

 as Mr. Jenner has justly remarked to me, makes it doubtful whether a similar 

 character can be relied on for distinguishing Desmidium quadrangulatum 

 from Desmidium Swartsii. The end view is triangular ; the sides are 

 slightly concave and the angles rounded and somewhat thickened. The en- 

 dochrome in this aspect has the rays equal in number to the angles, each ray 

 being usually bipartite. 



The variety /3. is an elegant microscopic object. Its foramina are large and 

 conspicuous. The only specimens I have seen were gathered by Mr. Side- 

 botham. Its characters are so distinct that it is unnecessary to compare this 

 variety with any other plant. 



A careless observer might mistake the triangular form for Desmidium 

 Swartsii. It differs however not only in the presence of foramina, but its 

 filaments are more slender and its joints longer in comparison with their 

 breadth. 



Several filamentous Desmidiese have no slight resemblance to Tcenice, and 

 none more than this species. 



Length of joint of a. (Dolgelley specimen), including connecting processes, 

 i-j^j'of an inch ; length between foramina ygVg- ; breadth of joint y,,^. 



