132 WISCONSIN PHYTOPLANKTON 



Ralfs grouped several species under A. incus so that the typical 

 A. incus of the early investigators is largely a matter of conjecture. 

 I have followed W. & G. S. West in considering the above described 

 alga the typical A. incus although it should be noted that their con- 

 ception of the species is based upon phylogenetic rather than upon 

 taxonomic grounds. 



A. incus belongs to the group of species with long spines and 

 straight-sided cells that includes A. triangularis Lagerheim and 

 A. quiriferus W. & G. S. West. It is distinguished from A. triangu- 

 laris by the shorter isthmus, straight apex of the semicell, and diver- 

 gent spines. 



var. EXTENsus Andersson. PI. 85, Figs. 23-26. 



Bih. KgL Sv. Vet.-Ak. Handl. 16, Afd. 3, No. 5: 13, pi. 1, fig. 7. 1890. Eichler 

 & Raciborski, Rozprawy Akad. TJmiej. w Krakowie 2 Ser. 6; 119, pi. 3, fig. 23. 

 1893. 



Arthrodesmus incus var. vulgaris Eichler & Eaciborski, Eozprawy Akad. Umiej. 

 w Krakowie 2 Ser. 6: 119, pi. 3, figs. 22, 24. 1893. 



Sinus widely open and semicircular to semielliptic, isthmus elon- 

 gate ; semicells trapeziform, with basal angles rounded and superior 

 angles sharper and with fairly short spines, spines parallel to diver- 

 gent, (Facultative planktont.) 



Cells 20-25 /t long ; breadth with spines 45-62 /x, breadth without 

 spines 15-17.5 fi ; breadth at isthmus 5-6 fi ; length of spines 15-22.5 /t. 



Bear (rrr). Devils (r), George (rrr), Horseshoe (rrr), Jones (rrr), Lindy (ss), 

 Lynx (rrr). Oak (rrr), Eudolph (rrr), Tank (r), "Wolf (rrr). 



This varietv is characterized by the elongate isthmus and the angle 

 at the base of the semicell, features which make the shape of the semi- 

 cells trapeziform instead of triangular. In Wisconsin specimens the 

 spines at the angles are usually subparallel instead of divergent. 



var. praelongus var. nov. PI. 85, Figs. 27-29. 



Cells larger, sinus trapeziform; semicells obversely triangular, 

 spines at the angles very long. (Euplanktont?) 



Cells 21-22.5 fj. long ; breadth with spines 61-65 /x, without spines 

 17.5-20 fi ; breadth at isthmus 5 fi ; thickness 9 jx ; length of spines 

 22-28 fx. 



Crab (rrr), Whitney (rrr). 



Very similar to the foregoing in the elongate isthmus but with 

 a sinus that has rounded and not angular sides. The spines are also 

 longer than those of other varieties of the species. The variety 

 might be considered a biradiate variety of Staurasirum cuspidatum 

 De Brebisson. 



