ARTHRODESMtfS. 95 



X. TETBACENTROTUM, Wolle. Plate XXII, figs. 8, 9. 



About as long as broad, smooth, constriction deep, sinus 

 enlarged outwardly ; semicells subreniforrn or subhexagonal, 

 base somewhat convex, ends broadly rounded ; basal angles on 

 eaeh side armed with a pair of subulate spines ; central pro- 

 tuberance low, over it sometimes, a series of bead-like granules. 



Diameter 3337 ,".. without spines; 60 p. with spines. 



Pond, Sussex Co., N. J, 



The form and charaeter of this species, is very near C. 

 antelopceum, but it is smaller and bean only two pairs of 

 aculei, not four or six. 



Genus, A.RTHRODESMUS, 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. convergens, (Ehrb.) Half's. Plate XXIII, figs. 19,20,21. 

 Semicells elliptic, each having its -pines curved towards 

 those of the other semicell. 

 Diameter 38-40 p. 



Several varieties are acknowledged to this specie.-, one with 

 shorter and almost straight spines, another with broadly 

 fusiform cells. Plate XXIII, figs. 22, 23, I place with the 

 varieties. 



This species is common. 



A. fragile Wolle. Plate XXIII, figs. 16, 17, 18. 



Semicells broad, oblong-oval ; aculei straight and parallel, 

 i. e., the aculeus of one semicell is parallel with that of the 

 other; deciduous. Chlorophyllous nuclei, two. 

 Diameter 38-33 p.; length somewhat less. 



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



A. Rauii, Wolle. Plate XXIII, figs. 13, 14, 15. 



Cells slightly longer than broad, aculated or verrucose; 

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

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

 two curved series of about six each on the membrane within 



