ARTHRODESMUS. 105 



A. INCRASSATUS, Lagh. Plate X, fig. 6. 



A large form, deeply constricted, sinus largely ampliated 

 outwardly; semi-cells subellipsoid, sides convex, ends highly 

 arched, superior angles surmounted each with a nearly 

 straight aculeus ; in middle two series of granules arched, 

 nearly parallel with the ends ; in vertical view rhomboid- 

 elliptic ; in lateral view somewhat circular ; membrane in 

 middle thickened and yellowish. 



Diameter of cells 40 //. ; length 50 ,u., without aculei. 

 Tewksbury, Massachusetts. 



Var. CYCLADATUS, Lagh. 



Somewhat smaller, eight granules in the middle of semi- 

 cell, one central and the others in circle around it. 

 This variety is near my Xanihidium tetracentrotum. 

 Tewksbury, Massachusetts. 



A. NOTOCHONDRUS, Lagh. Plate X, figs. 4, 5. 



In size and constriction like A. incrassatm; semi-cells sub- 

 semicircular; sides convex, ends straight, with* margins mod- 

 erately granulate ; angles armed with a long divergent spine; 

 semi-cells in vertical view elliptic, with three longitudinal 

 series of granules ; in lateral view subcircular. Membrane 

 smooth, in middle somewhat thickened and yellowish. 

 Diameter of cells, without spines, 30 jn. ; length 32 //. 

 Tewksbury, Massachusetts. 



A. TRIANGULARIS, Lagh. Plate X, figs. 14, 15. 



Sinus between the semi-cells wide, obtuse; semi -cells tri- 

 angular, connected by a cylindrical isthmus, ends somewhat 

 retuse in middle; angles subacute bearing a long straight 

 spine, either parallel or slightly converging ; semi-cells in 

 vertical view lanceolate-oval ; in lateral view subcircular. 

 Membrane hyaline, in middle not thickened. 

 Diameter, without spines, 25 ^. ; length 30 /*. 

 Georgia. 



A. PACHYCEROS, Lagh. Plate X, figs. 12, 13. 



Cells small, constriction an acute-angled sinus ; semi-cella 

 suboval, sides convex, end arched, angles obtuse, each bear- 

 ing a firm spine, straight or slightly bent, diverging ; semi- 

 cells in vertical view oval ; in lateral view subcircular. 

 Membrane smooth. 



Diameter, without spines, 18 yw. 



Properly a Cuban plant. Not sure that it was found within 

 our borders. 



