134 BRITISH DESMIDIACE^l. 



a considerable range of variation. There is a thickening of 

 the cell-wall at the basal angles of the seniicells which com- 

 monly gives them a submamillate appearance. 



Some of the tropical forms attain a large size : length 57 

 -64 n ; breadth 67-75 JJL ; breadth of isthmus 30-35 JJL ; thick- 

 ness 36 /u. In these specimens the thickening of the basal 

 angles is generally prominent,, and passing through it is a 

 large conical pore, the wider end of which is towards the 

 interior of the cell-wall. We give a figure of one of these 

 specimens from Singapore to show this character (PI. LVI, 

 fig. 4). Sometimes the mamillate thickening becomes quite 

 hyaline with the exception of the conical pore, which then 

 stands out very prominently, and has the appearance of a 

 short spine attached to the basal angle. Turner figured this 

 pore as a spine at the angle, and he put forward the name 

 i{ Cosmarium palustre '' for typical specimens of the large 

 form of C. obsoletum which is so abundant in tropical Asia. 

 It is in old individuals in which the cell-wall becomes of a 

 yellow or yellow-brown colour that this " spinate " appearance 

 is most marked. Sometimes the actual thickening of the 

 basal angle has the form of a bluntly-conical mucro. The 

 large tropical forms have recently been named " var. Sitvense' 1 

 by Gutwiiiski (' Alg. Ins. Java/ 1902, p. 594, t. 38, f. 39), and 

 so far as is known they are characteristic of the Lido-Malay 

 region. 



The British specimens are usually of small size and they 

 show 110 trace of the pore which is present in the basal angles 

 of the large tropical forms. The cell-wall of the smaller 

 forms is finely punctate, but that of the larger forms is some- 

 what sparsely and finely scrobiculate. 



2. Cosmarium Smolandicum Lund. 

 (PL LVI, fig. 5.) 



Cosmarium Smolandicum Lund. Desna. Suec. 1871, p. 39, t.2, f. 17 ; Wolle, 

 Desm. U.S. 1884, p. 66, t. 16, f. 35, 36 ; De Toni, Syll. Alg. 1889, p. 984 ; 

 Nordst. Index Desm. 1896, p. 235. 



Ursinella Smolandica Kuntze, Eevis. gen. plant. 1891, p. 925. 



Cells of medium size, subcircular, a little longer 

 than broad, very deeply constricted, sinus somewhat 

 narrow and linear ; semicells subsemicircular, apices 

 convex but sub truncate in the middle, basal angles 

 obtuse and furnished witli a prominent papilla. Side 



