BRITISH DESMIDIE^:. 67 



2. S. excavatmn (Ralfs) ; joints longer than broad, having a deep sinus 

 ou both sides, and two sessile glands on each margin at their 

 junction. 



Schistochihnn exeavatum, Ralfs, m Jenner's Fl. of Tunbridge Wells, p. 192 



(18-15). 

 Spharozosma excavata, Ralfs, in Annals of Nat. Hist. v. 16. p. 15. t. 3. f. 8. 



(1845); Trans, of Bot. Society of Edinburgh, v. 2. p. 168. Hass. 



Brit. Freshwater Jig. p. 349. 



Hab. Pools. Dolgelley and Penzance, J. i2. Cross-in-hand, and Ashdown 

 Forest, Sussex ; bogs at Fisher's Castle, Kent ; Farnham, Surrey, and near 

 Southampton, Mr. Jenner. Bristol, Mr. Broome. iVmbleside, Mr. Side- 

 botham. Rochdale, 3Ir. Coates. 



Falaise, Brebisson. West Point, New York, Baileij. 



\eTy minute, seldom more than twenty-five joints in the filament, which is 

 fragile and finally separates into single joints ; at their junction, in the front 

 view, are two minute glands or processes, situated one near each angle, and 

 nearly im"isible before the escape of the endochrome. The joints are nearly 

 twice as long as broad and much constricted in the middle ; the constriction 

 is hke an excavation or broad sinus on each side, so that the margins of tlie 

 filament appear sinuated. The endochrome is pale bluish green with minute 

 scattered granules. 



The transverse view is oblong with four sessile glands, two on each side, 

 and situated near the ends. 



From their pale colour and minute size I have experienced much difficulty 

 in determining the form of the angles, which in some specimens seem to me 

 entire, but in others emarginate. Mr. Jenner, using a more powerful mi- 

 croscope, informs me that each is apparently furnished with three minute 

 teeth, which can be detected only when favourably situated for observation. 

 On each segment of the empty joint he finds three transAcrse series of minute 

 granules, whose appearance at the margins produces this mmutely toothed 

 appearance. 



I have frequently gathered this species, but always in small quantity, mixed 

 with other Desmidiese ; I am therefore miable to decide whether it possesses 

 a mucous sheath. 



The sporangia of S. exeavatum have been gathered near Farnham in Surrey, 

 and at Cross-in-hand, Sussex, by Mr. Jenner, who sent his specimen to me for 

 examination. The joints after separation couple ; the sporangium is situated 

 between them, and is large compared with the conjugating cells. It is elliptical, 

 and the empty cells are closely connected with it, one at each end. As the 

 specimen was mounted, I was prevented from using the triplet ; but from 

 Mr. Jenner' s description, the conjugation takes place from their flat or front 

 surfaces, and not in the usual manner from one of the lateral sinuses ; therefore 

 either one of the ends or one of the lateral margins will be presented to the eye. 



Length of joints ^5^ 5 of an inch ; breadth of segments 3^'-^ ; breadth at 

 constriction ^Vo ^ length of sporangium y^ ; breadth from -^^ to ^pgL^. 



F 



