Mr. J. Ralfs on the species of Desmidium. 375 



Foreign specimens given me by Mr. Berkeley exactly agree 

 with the above description. 



** Filaments without a mucous sheath. 



3. D. Swartzii, Ag. Filaments triangular ; joints shorter than broad. 

 PL VIII. fig. 3. Grev. Crypt. Fl. t. 292 ; Ehren. Die In/us. p. 140. 

 t. 10. fig. 8 ; Pritch. In/us. p. 183. 



Tunbridge Wells, Mr. Borrer ; Swansea, S. Wales ; Caernarvon 

 and Dolgelley, N. Wales ; Penzance. 



Agardh seems not to have understood this species. In the 

 'Conspectus criticus Diatomacearum' he says, "filis planis tri- 

 angulisve," and afterwards, " Fila mihi plana visa, secundum Car- 

 michael revera triangula sunt ; secundum Lyngbye articuli soluti 

 cito figuram triangulam induunt. Iterum itaque hoc respectu 

 observanda." 



Dr. Greville's description is very correct : the filaments are 

 no doubt triangular, as a little care in using the microscope may 

 easily determine. Under the lens two angles always appear at 

 the two margins of the filament. The third angle is marked by 

 the dark line caused by the greater thickness in that part. As 

 the filaments are twisted, the dark line regularly passes, as seen in 

 the sketch, from one margin to the other. By examining the fila- 

 ment where the angle (shown by the dark line) touches the mar- 

 gin and bringing the lens a little nearer, and then withdrawing 

 it again, both angles may be distinctly seen. 



Transverse view triangular, the angles blunt, the sides slightly 

 concave ; endochrome in three portions placed in the angles, and 

 connected in the centre by three processes resembling stalks. But 

 this is only seen when the joints separate spontaneously. 



When a joint is separated under the microscope, a cloud of 

 extremely minute granules is poured out, which for an instant 

 obscures the view. 



It adheres but slightly to paper, is very fragile, but separates 

 spontaneously into single joints with less readiness than D. mu- 

 cosum and D. cylindricum. 



Mr. Borrer has kindly presented me with a portion of a spe- 

 cimen given him by Mr. D. Turner as the Conferva dissiliens of 

 Dillwyn, but which is undoubtedly the plant now described. 



4. D. Borreri. Filaments cylindrical, inflated, with two angles ; 

 joints about twice as long as broad. PL VIII. fig. 4. 



I have great pleasure in dedicating this species to Mr. Borrer, 

 who directed my attention to it in a boggy ditch at Cwm Bychan 

 in July last, when I accompanied him in a very agreeable excur- 

 sion among the Welsh mountains. 



We afterwards found it above Twll Du in a small pool near 



