84 THE AMERICAN MONTHLY [Mar., 



Outer rings concentric, inner convoluted ; 20-40 bristle-shaped spines. 



setigera. 



All rings in a double spiral ; 20-30 different spines echinastrum. 



All rings in a half spiral ; 8 conical spines clavata. 



69. STYLOCHLAMYDIUM. 



Rings partly concentric, partly spiral ; centreof disk spongy spongiosum. 



70. HYMENIASTEUM. 



Arms 2j times as long as broad, witli terminal spine ternarium. 



71. EUCHITONIA. 



Odd arm li times as long as paired arms ; patagium enveloping arms. 



stohrii. 



Odd arm somewhat larger than paired arms ; patagium nearly envel- 

 oping arms luulleri. 



Arms nearly alike ; patagium enveloping about g of arms triaugulum. 



72. STAURALASTEUM. 



Arms nearly spherical at ends, edges parallel antiquum. 



Arms truncated at ends, edges divergent staurolonche. 



73. HISTIASTRUM. 



Arms at truncated end a little broader than at base ; 1 terminal spine. 



quaternarium. 



Arms at truncated end 3 times as broad as at base ; 1 terminal spine. 



gladiatum. 



Arms at nearly spherical end 3 times as broad as in linear part ; 5 ter- 

 minal spines.. coronatum. 



Arms in egg-shaped half 3 times as broad as in linear half; 10-12 ter- 

 minal spines circulare. 



74. STEPHANASTRUxM. 



Ends of arms connected by ribbon shaped patagium rhombus. 



75. PENTINASTRUM. 



Arms globose at ends; patagium complete goniaster. 



76. SPONGODISCUS. 



No concentric rings; numerous radial beams resnrgens. 



Eleven-twelve concentric rings; no radial beams spongooyclia. 



77. SPONGOTRIPUS. 



Spongy disk circular ; radial spines pyramidal neumayri. 



78. STYLOTROCHUS. 



Eight-twelve long, bristle shaped marginal spines arachnius. 



Sixteen-twenty short marginal spines craticulatus. 



