AR^EOSOMA 175 



and primary tubercles. Only rarely is the relative size of the actinostome 

 or abactinal system of any importance, but the relative amount of calci- 

 fication of the coronal plates and the degree to which they are bent adorally 

 are often very useful characters. The species which seem to us probably 

 valid may be distinguished as follows : 



Actinal primary tubercles not forming an uninterrupted marginal series at 



ambulacral edge of each half-interambulacrum, since some inter- 



ambulacral plates (usually every other one, at least near ambitus) 



do not have a primary tubercle at extreme outer end. 



Abactinal primary tubercles fewer than 100, nearly or quite as large as 



those of actinal surface thetidis. 



Abactinal primary tubercles more than 200, much smaller than those of 



actinal side bicolor. 



Actinal primary tubercles forming an uninterrupted marginal series at 

 ambulacral edge of each half-interambulacrum, since each inter- 

 ambulacral plate has a primary tubercle at its extreme outer end. 

 Ambulacra very narrow, about .40 of interambulacra ; primary spines 



near ambitus banded with red or reddish-purple pellucidum. 



Ambulacra more than half as wide as interambulacra ; primary spines 

 not banded. 

 Ambulacra very broad, more than .75 of interambulacra. 



Ambulacra .90 of interambulacra; interambulacral plates 

 much more numerous abactinally than actinally, strongly 

 inclined (near ambitus, curved) towards mouth, those at 

 ambitus with inner end so much more adoral than outer, 

 that the plate is 30 per cent longer than one-half the width 



of interambulacrum eurypatum. 



Ambulacra .80 of interambulacra; interambulacral plates 

 nearly as numerous actinally as abactinally, more or less 

 oblique, but even at ambitus their length is only about 10 

 per cent longer than one-half the width of interambu- 

 lacrum leptaleum. 



Ambulacra moderately broad, .50-.75 of interambulacra. 



Entire test bright red, the color more or less well-preserved 

 in alcoholic and dry specimens; large tridentate pedi- 

 cellarise with curved valves having the blade strongly 

 involute except at tip. 

 Test bright rose-red; actinostome about .25 h. d.; few ac- 

 tinal interambulacral plates with two or more primaries hystrix. 

 Test bright vermilion ; actinostome less than .20 h. d. ; 

 most actinal interambulacral plates with two or more 



primaries pyrochloa. 



Test never bright red; colors variable, the actinal and 

 abactinal surfaces often different ; colors usually more 

 or less bleached or altered in preserved specimens. 

 Ambulacral plates rather high and few, interambulacral 

 about four-fifths as many ; test reddish-purple above, 

 lighter, often yellowish, beneath Belli. 



