110 



WISCONSIN PHYTOPLANKTON 



46. Staurastrum cerastes Lundell. PL 80, Figs. 8-10. 



Nova Acta Eeg. Soe. Sci. Upsaliae 3 Ser., 8: 69, pi. 4, fig. 6. 1871; W. & G. S. 

 West, Trans. Linn. Soc. 2nd. Ser. Bot. 5: 268, pi. 18, fig. 4. 1896. 



Cells of medium size, breadth and length about equal, slightly con- 

 stricted, sinus a slight wedge-shaped constriction, isthmus relatively 

 broad; semicells trapeziform-rectangular, tapering only slightly to- 

 wards the isthmus, dorsal margin rounded, angles continued in stout 

 tapering hollow processes with truncate ends that terminate in three 

 or four minute teeth, processes strongly convergent ; base of semicells 

 with conical papillae or mucros just above the isthmus, one usually 

 lying midway between each adjacent pair of processes, rarely with 

 the mucros directly below the processes; median portion of semicell 

 with two transverse rows of bifid emarginate verrucae that are con- 

 tinued in the processes as a single row of conical verrucae; apex of 

 semicells with emarginate verrucae that continue for a ways along 

 the dorsal margin of the processes and then give place to simple 

 conical verrucae; ventral margin of processes with simple verrucae. 

 Vertical view quadrangular (in Wisconsin specimens) with the sides 

 retuse and the angles continued in stout straight processes that grad- 

 ually taper to a truncate end; central area of the cell body with a 

 rectangle of twelve emarginate verrucae (three on each side) whose 

 sides parallel those of the cell, angles of this design with two parallel 

 rows of emarginate verrucae leading to the bases of the processes and 

 then giving way to a double or to a single row of conical verrucae 

 than continues to the end of the processes ; lateral margins of cells 

 with emarginate verrucae; lateral margins of processes with short 

 outwardly divergent spines. ( Tychoplanktont. ) 



Zygospores unknown. 



Cells 50 fi long ; breadth with processes 45-52 ju,, without processes 

 25 ju, ; breadth at isthmus 11 ju.. 



Found (rrr), Rudolph (rrr), Speese (rrr). 



The alga as found in Wisconsin is always quadrangular although 

 triangular specimens are well known in Europe. There were also 

 but four supraisthmal mucros in all individuals examined. These 

 resemble, in this respect, the type figured by Lundell and not the 

 type with a transverse row of mucros above the isthmus that W. & 

 G. S. West show. Welsh plankton material examined in the G. S. 

 West collection also had the transverse row of conical papillae at the 

 base of the semicells. It is possible that a separation of forms can 

 be made on this feature of basal ornamentation but for the present 

 the two are considered identical. 



47. Staurastrum arachne Ralfs. 



Ann. & Mag. of Nat. Hist. 15: 157, pi. 11, figs. 6A-6B. 1845; Brit. Desm. 136, 

 pi. 23, figs. 6A-6B. 1848. 



Cells fairly small, length about half the breadth (with processes), 

 deeply constricted, sinus widely open from a subacuminate apex, 



