ARTHRODESMUS. 35 



X. TORREYI, Wolle. Plate LVI, figs. 13, 14. 



Nearly equal in length and breadth ; semi-cell somewhat 

 hexagonal, half as long as wide ; superior and lateral angles 

 slightly protruding and surmounted each by a firm, straight, 

 or slightly bent aculeus ; in vertical view elliptic with one 

 aculeus on, and another within, the margin of each end ; on 

 each side a rounded prominence peculiar to the genus. 

 Lateral view elliptic with a constriction in the middle, 

 two aculei at each end and two short tips evident near 

 the middle. 



Diameter 33 yw, without aculei; with them 65 //; length 75 /<. 



Ocean County, New Jersey. First found in Horicon 

 Lake, near the home of the venerable brother of the late 

 John Torrey, of botanical fame. 



Genus, ARTHKODESMUS. V. Des. U. S. p. 95. 



A. INCRASSATUS, Lagh. Plate LXI, 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 ; mem- 

 brane in middle thickened and yellowish. 



Diameter of cells 40 ^ ; length 50 //, 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 XantMdium tetracentrotum, Des. 

 U. S. p. 95. 



Tewksbury, Massachusetts. 



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



In size and constriction like A. incrassatus; 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. Men> 

 brane smooth, in middle somewhat thickened and 



Diameter of cells, without spines, 30 u: length 32 

 Tewksbury, Massachusetts. 



