52 WISCONSIN PHYTOPLANKTON 



var. GRAciLLiMUM G. M. Smith. PL 64, Fig. 1. 

 Trans. Wis. Acad. Sei., Arts & Lett. 20: 344, pi. 9, figs. 6-11. 1922. 



Cells slightly larger than the type; base of the polar lobe much 

 narrower; lateral lobes not incised, with parallel sides and apices 

 furcate; horizontally disposed bases of the semicells only slightly 

 tapering. (Facultative planktont.) 



Cells 160 ju, long; greatest breadth 130 /i, polar lobe 65 /i, broad at 

 apex ; isthmus 17.5 /x broad, 



Lindy (rrr), Little Doctor (rrr). 



Distinguished from the foregoing by the much narrower base of 

 the polar lobe and by the nature of the lateral margins. The basal 

 portion of the semicell is likewise much narrower than the basal por- 

 tion of the semicell in the type, this narrowness giving the cell a 

 more delicate appearance. In this variety no case has been observed 

 where there is an incision of the lateral lobes. 



12. MicRASTERiAS AMERICANA (Ehreuberg) Ralfs. PI. 63, Fig. 5; 



PI. 64, Fig. 2. 



Brit. Desm. p. XIX. 1848; WoUe, Desm. U. S. 112, pi. 32, fig. 2. 1884; W. & 

 G. S. West, Monogr. Brit. Desm. 2: 117, pi. 53, figs. 4-5; pi. 54, figs. 1-3. 1905. 

 Micrasterias morsa Ralfs, Brit. Desm. 74, pi. 10, figs. lA-lE. 1848. 



Cells fairly large, deeply constricted, sinus acute-angled and with 

 apex acuminate, isthmus narrow; semicells five-lobed, incisions be- 

 tween polar and lateral lobes broad and with blunt apices; incisions 

 between lateral lobes narrowly fusiform and with apices rounded; 

 polar lobe with lateral margins parallel at base and then outwardly 

 divergent to form a cuneate apex ; apical margin of polar lobe broadly 

 retuse and with lateral margins forming upwardly divergent processes 

 with slightly convergent sides and truncate-emarginate apices; apex 

 of polar lobe with a second similar but smaller process arising near 

 the base of each of the lateral processes, secondary processes less out- 

 wardly divergent than the primary processes and asymmetrically 

 disposed, one on each side of the polar lobe; lateral lobes with a 

 shallow broad incision dividing them asymmetrically, the lobules ad- 

 jacent to the polar incision or to the sinus being longer than those 

 adjacent to the median lateral incision ; all lobules with ends truncate- 

 retuse and with 3-4 acute or rounded denticulations ; portion of 

 lobules adjacent to the secondary incisions, apical polar incisions and 

 sometimes the portion of the cell bordering the polar incision with 

 marginal denticulations ; semicell body with two fairly conspicuous 

 blunt teeth at the base of the polar incisions, sometimes with a central 

 supraisthmal protuberance bearing scattered granules ; polar and 

 lateral lobes with a few scattered granules. Vertical view rhomboidal, 

 with poles truncate and polar lobe showing the asymmetrical position 

 of the processes. Lateral view of the semicells narrowly pyramidate, 

 with bases rounded and apices continued in four short processes. 

 Chloroplast extending into all five lobes and with the portion in the 

 polar lobe fairly deeply incised, with two vertical ridges extending 



