92 DESMIDS OF THE UNITED STATES. 



T. GIGANTEUS, Wood. Plate XX. fig. 6. 



Very large, oblong, three times Longer than broad, with the 

 ends not usually attenuate but broadly rounded; suture pro- 

 found, linear; cytioderm irregularly granulately punctate; 

 somewhat plicate at the base of the semicells. 



Diameter 75 a. more or less. 



Ponds, Pennsylvania, and New Jersey. 



The diameter varies from 68-75 //. and " is preserved uni- 

 form until near the end, where there is an alteration in the 

 line of the margin so as to cause some contraction, which is 

 however wanting in some specimen-." 



Genus, XANTHIDIUM, Ehrb. 



Cells single or geminately concatenate, inflated, profoundly 

 constricted; Bemicells compressed, entire, spinous, protruding 

 in the centre as a rounded, truncate, or denticulate tubercle. 

 Cytioderm firm, the Bpines with which it is armed, simple or 

 bi-tri-furcately divided at the ends. Zygospore globose, 

 smooth or Bpinous. 



X. armatum, (Breb.). Ralfs. Plate XXI, figs. 1-4. 



Semicells largest al the base, about as long a- broad, armed 

 with numerous, short, stout spines terminated by two, three or 

 more diverging point-. 



Diameter 62-140/1. 



Very variable in size. Fig. 1 is a finely developed form 

 from Mt. Everett, Mass. The other two are more usual 

 varieties found in all the States with which I had any com- 

 munication. 



Tins is the only species with Bpines divided at the apex. 

 Wood has two more, as X. aretiscon, and X. coronatum, but 

 these must be separated from this genus, and placed with 

 Stauragtrum. 



The following have subulate spines: 



X. \. [JLEATUM, (Ehr.) Breb. Plate XXIII, figs. 10, 11, 12. 



Spines subulate, more or less scattered; central projection 

 truncate, obscurely dentate. 

 Diameter 62-70 fi. 



Prof. Bailey reports this species from South Carolina, 

 Georgia and Florida. 



