Mr. J. Ralfs ow the species o/' 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. 

 PI. 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. W^ales ; Penzance. 



Agardh seems not to have understood this species. In the 

 'Conspectus criticus Diatomacearum' he says, ^'^tilis planis tri- 

 angidisve/' and afterwards, " Fila mihi plana \'isa, secundum Car- 

 michael revera triangula sunt ; secundum Lpigbye articuli soluti 

 cito figm-am triangrJam induunt. Iterum itaque hoc rcspectu 

 observanda." 



Dr. Grexille^s description is veiy correct : the filaments arc 

 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 toasted, the dark line regularly passes, as seen in 

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

 ment where the angle (sho^Ti by the dark hne) touches the mar- 

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

 it again, both angles may be distinctly seen. 



Transverse \iew triangular, the angles bkmt, the sides slightly 

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

 connected in the centre by three processes resembhng stalks. But 

 this is only seen when the joints separate spontaneously. 



^Tien a joint is separated under the microscope, a cloud of 

 extremely minute granules is pom-ed out, which for an instant 

 obscures the %dew. 



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

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

 cosum and D. cylindricum. 



Mr. Borrer has kindly presented me vd\h a portion of a spe- 

 cimen given him by jMr. 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. PI. VIII. fig. 4. 



I have great pleasure in dedicating this species to Mr. Bon-er, 

 who directed my attention to it in a boggy ditch at C^\Tn Bychan 

 in July last, when I accomjjanied him in a very agreeable excui*- 

 sion among the Welsh mountains. 



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



