WISCONSIN PHYTOPLANKTON 61 



spines on the apices of the seniicells. These may range from a single 

 extra apical spine on one of the two semicells (Figs. 10) to an extra 

 spine at the lateral margin of both semicell apices. The highest 

 development of this variation observed is shown in Figure 12 where 

 there are two accessory spines arising at the same point. This sug- 

 gests very strongly a transition to X. Jiastiferum Turner, but the 

 great preponderance of typical X. suhJiastiferum in the collection 

 shows that these specimens are not reduced forms of X. hastiferum. 

 The other type of variation is a reduction of the two spines at the 

 lateral margin to a single spine. In most cases but one lateral margin 

 of the semicell shows this reduction. In the rarer cases where the 

 spines on both lateral margins of a semicell are reduced to a single 

 spine the semicells have much the appearance of Arthrodesmus 

 convergens Ehrenberg. 



var. TOWERi (Cushman) G. M. Smith. PI. 66, Figs. 13-18; PI. 67, 



Figs. 1-3. 



Trans. Wis. Acad. Sci., Arts & Lett. 20: 346, pi. 10, figs. 9-13. 1922. 



Semicells obversely subtriangular, sinus more open than in the 

 type; ventral margin straight or curved, when curved much more 

 tumid than the dorsal margin ; spines much longer in proportion to 

 the breadth of the semicell. (Euplanktont.) 



Cells 45-75 fx long with spines, 40-47 /a long without spines ; 75-90 /a 

 broad with spines, 39-45 fj. broad without spines ; isthmus 10-12 [i 

 broad ; thickness 24r-26 [x ; spines 17.5-25 fx. 



Carroll (s), Horseshoe (rrr). Shell (cc). Two Sisters (rrr). 



This variety differs from the type in the greater proportionate 

 length of the spines and in the difference between the dorsal and 

 ventral margins of the semicells. There are the same variations in 

 the presence of accessory apical spines or in the reduction of spines 

 that are found in the type. The collections from Carroll lake were 

 particularly interesting since they exhibited an almost complete series 

 of transitions from the normal to a form that looks very much like 

 Arthrodesmus or certain biradiate Staurastrum species. 



var. Johnsonii (W. & G. S. West) comb. nov. PL 66, Figs. 4r-7. 



Xanthidium hastiferum var. Johnsonii W. & G. S. West, Jour. Linn. Soc. Bot. 

 33: 299. 1898. 



Xanthidium antilopaeum, var. javanicum Johnson non Nordstedt, Bull. Torr. 

 Bot. CI. 22: 295, pi. 239, fig. 10. 1895. 



Semicells subsemicircular to subhexagonal, ventral margin with 

 straight to tumid sides, apical margin strongly tumid, at times with 

 median portion somewhat elevated ; lateral margins with two, fairly 

 stout, straight to strongly curved spines that lie in the same vertical 



