126 WISCONSIN PHYTOPLANKTON 



KEY TO THE SPECIES. 



Semicells with two spines. 



Shape of semicells approximately elliptic. 

 Apex of semicells a smooth cui've. 



S2:)lnes short and convergent (1) A. convergens 



Spines long, parallel to divergent (3) A. subulatus 



Apex truncate in median portion (4) A. michiganensis 



Shape of semicells not elliptic. 



Diameter of semicells greater than length of spines. 



Isthmus elongate (6) A. constrictus 



Isthmus not elongate. 



Spines convergent (7) A. Balfsii 



Spines divergent (5) A. pMmus 



Diameter of semicells less than length of spines. 



Apex of semicells retuse (8) A. quiriferus 



Apex of semicells not retuse. 



Semicells subrectangular (2) A. Bulnheimii 



Semicells subtriangular. 



Apex flat (9) A. incus 



Apex convex (10) A. triagularis 



Semicells with four spines (11) A. octocornis 



1. Arthrodesmus convergens Ehrenberg. PL 84, Fig, 6. 



Die Infusionsthierchen 152, pi 10, figs. 18A-18B. 1838 ; Ealfs, Brit. Desm. 118, 

 pi. 20, figs. 3A-3D. 1848; G. S. West, Jour. Linn. Soc. Bot. 34: 397, text fig. 4. 

 1899; W. & G. S. West, Monogr. Brit. Desm. 4: 106, pi. 116, figs. 4-13. 1912. 



Cells of medium size, breadth (without spines) slightly greater 

 than the length, very deeply constricted, sinus narrowly linear and 

 with outer portion widely open, isthmus narrow; semicells trans- 

 versely elliptic, with the apex more rounded than the base of the 

 semicell, lateral margins broadly rounded and furnished with a single 

 stout tapering spine, spines strongly convergent and with the ex- 

 tremities of opposite pairs frequently apposed. Vertical view nar- 

 rowly elliptic, with a short spine at each pole. Chloroplast axial, 

 with a single conspicuous pyrenoid. 



Zygospores spherical, with a smooth wall (W. & G. S. West). 



Cells 30-45 fi long ; breadth with spines 45-75 /x, wdthout spines 

 32-48 IX ; isthmus 10-11 /j, broad ; spines 12-16 /a long. 



Diameter of zygospores 39-48 [x (W. & G. S. West). 



Diamond (rrr), Muskallonge (rrr). 



Variation is much more pronounced in this species than in any 

 other of the genus. This is particularly true of the spines and it is 

 not at all unusual to find those of one semicell reduced or even en- 

 tirely lacking. No particular significance can be attached to this 

 suppression of spines since when new semicells are formed they are 

 generally quite typical. 



The species is the largest of the genus and is characterized by the 

 elliptic semicells and the short converging spines. It has a certain 



