BRITISH DESMIDIEjE. 63 



lines running from side to side and crossing each other : the additional line of 

 course depends on the additional angle. When these lines approach the 

 opposite margins of the filament only one side is presented to the eye, and 

 the filament is then of the same hreadth as in D. Swartzii, but as it is regu- 

 larly twisted its apparent breadth varies, being greatest where both dark 

 lines cross each other in the middle. The end view also has one angle more, 

 and therefore the endochrome exhibits four instead of three rays. I may 

 however observe, that the teeth, as Mr. Jenner has jjointed out, are rounded 

 in my specimens of D. quadra7igulatum and angular in B. Sioartzti; but 

 further experience must determine whether this character is constant. 



D. quadrangulatum requires about 40 joints to complete a turn, or before 

 the same angle again appears at the same margin. 



It aflPords some sanction to this arrangement, that shortly after my notice 

 of this plant appeared hi the ' Annals of Natural History,' it was described 

 by Kiitzing in his ' Phycologia Germanica' as a distinct species. 



Length of joint -^^ of an inch; least breadth or side in transverse view 

 ^i-3 ; greatest breadth, or diagonal, in transverse view ^\-^. 



Tab. V. a. portion of a filament ; b. filament less magnified ; c. empty 

 joint ; d, e. transverse views. 



4. APTOGONUM. 



Filament elongated, triangular or plane ; joints bicrenate at the mar- 

 gins ; an oval foramen between the joints. 



The filaments are elongated, jointed, fragile, and either triangular 

 or plane; the joints at the angles are bicrenate, and at their junction 

 are excavated at the centre, and thus form a series of oval foramina. 

 The endochrome is bipartite. 



By Brebisson, Meneghini and Kiitzing this plant is included in 

 Desmidium, and certainly the state with triangular filaments does at 

 first sight appear closely allied to Desmidium Swartzii ; but the large 

 oval foramen between the joints is so remarkable a character, that I 

 must concur with Ehrenberg in placing it in a separate genus. As 

 however Ehrenberg's name, Odontella, had previously been given by 

 Agardh to some Diatomaceae, it became necessary to find another 

 name. 



Ehrenberg included Spharozosma in his Odontella^ but the present 

 genus is essentially distinct from Sphcerozosma ; in the latter the joints 

 have their margins incised or sinuated, and gland-like processes at 

 their junctions, and it is merely by the interposition of these processes 

 that the joints are now and then slightly separated. In this genus. 



