ARTimODESMUS. 99 



Forma triquetra nob. (PI. CXXVIII, fig. 1G.) 



Staurastrum dcjectum Hreb. subsp. Tellamii W. & G. S. West, New Brit. 

 Freshw. Alg. 1894, p. 11, t, 2, f. 45 ; Alg. S. England, 1897, p. 492. 



Vertical view triangular with retuse sides. 

 Length (without spines) 21 p.; breadth without spines 

 21 ju,, with spines 48 jn; breadth of isthmus 6*5 /x. 

 ENGLAND.- -Gun wen Moor, Cornwall ! 



This Desmicl lias little in common with Staurastrum dejectum, 

 whereas in front view ifc agrees very closely with some forms 

 of A. triangularis. It is in transitional forms of this kind 

 that the distinction between the genera Arihrodesmus and 

 Staurastrum breaks down. The occurrence of such forms, 

 however, does not seriously interfere with the view that it is 

 expedient to retain the genus Arthrodesmus. 



Var. inflatus W. & G. S. West. (PI. CXIV, figs. 14- 



15.) 



A. triangularis forma West, Alg. W. Ireland, 1892, p. 168, t. 24, f. 19. 

 A. triangularis var. inflatus W. & G-. S. West, Some Desm. U. S. 1898, 

 p. 320 ; Cushman in Khodora, vii, 1905, p. 261. 



Semicells much inflated, angular-elliptic, apices 

 elevated and generally slightly truncate or faintly 

 retuse in the middle, more rarely entirely convex ; 

 spines stronger and very slightly divergent. Isthmus 

 very shortly cylindrical. 



Length 24-38 /x ; breadth without spines 19-25^, 

 with spines 57-72 /x; breadth of isthmus 5'8-7'5/x; 

 thickness 10'5-12'5/x. 



WALES.- -Llyn Idwal, Carnarvonshire ! 



SCOTLAND.- -Bressay, Shetlands ! 



Geogr. Distribution.- -United States. 



This form occurs chiefly in the plankton and is readily 

 distinguished by its inHated semicells. 



In the plankton of Cwellyn, Carnarvonshire,, some large 

 forms with longer and stouter spines have been observed. 

 These we have named " forma ROBUSTA." Length 32-37 /UL ; 

 breadth without spines 26'5-27'5ju, with spines 100-105^; 

 breadth of isthmus 8-8'7/i. (PI. CXIV, fig. 16). 



