ARTHRODESMUS. 103 



Genus, AETHEODESMUS, Ehrb. 



Cells simple, compressed, deeply constricted in the middle ; 

 semi -cells broader than long, with a single spine or mucro on 

 each side, but otherwise smooth and entire. 



A. CONVERGEXS, (Ehrb.), Ealfs. Plate XXVI, figs. 19, 20, 21. 



Semi-cells elliptic, each having its spines curved towards 

 those of the other semi-cell. 

 Diameter 38-40 /<. 



Several varieties are acknowledged to this species, one with 

 shorter and almost straight spines, another with broadly fusi- 

 form cells. Plate XXVI, figs. 22, 23, 1 place with the varieties. 

 This species is common. 



A. KRAGILIS, Wolle. Plate XXVI, figs. 16, 17, 18. 



Semi -cells broad, oblong-oval; aculei straight and parallel, 

 /. e., the aculeus of one semi-cell is parallel with that of the 

 other ; deciduous. Chlorophyllous nuclei, two. 

 Diameter 33-38 /A; length somewhat less. 



Central New Jersey, pond at Hammonton, and other waters. 



A. BAUII, Wolle. Plate XXVI, figs. 13, 14, 15. 



Cells slightly longer than broad, aculeated or verrucose ; 

 aculei short and stout, deciduous, leaving, after falling off, 

 large verrucae ; usually six on the margin of each end, and 

 two curved series of about six each on the membrane within 

 margin ; semi-cell broadly elliptic, with a single outwardly 

 curved aculeus at each end. 



Diameter of cell without aculei 38 /^. ; with aculei 63 p. 



This species was collected with swamp moss (Sphagnum], 

 by E. A. Rau, in a pond near Newfield, N. J. It bears some 

 resemblance to A. divergens, Rab., but it is not subtillissime 

 verrucolosus ; also to A. quadridens, Wood, but it is twice the 

 size of that plant, and not quadridens. 



A. OVALIS, Wolle. Plate XXVII, figs. 13, 14. 



Cells small, smooth, often about one-fourth longer than 

 wide ; semi-cells oval, armed at each end with a straight or 

 diverging aculeus. 



Diameter 20 ju., without aculei. 

 Ponds, Mt. Everett, Mass. 



The smaller size, the straight, erect or slightly diverging 

 spines, I consider sufficient to separate this form from A. 

 convergent. 



