ECHINUS. 263 



armatus, the globiferous pedicellarise are used as the distinguishing mark of that 

 species. 



Ambulacral plates, at least at and below the ambitus, each with a primary 



tubercle. 

 Test rather low, often depressed, vertical diameter rarely exceeding .55 



horizontal. 



Ambulacral plates few (18-20 in specimens 45-50 mm. h. d., 24-28 in 

 specimens 65-100 mm.); valves of large tridentate pedicel- 

 lariae, broad and flattened. 

 Test not specially thick or rough; v. d. about equal to half h. d.; 



abactinal system much smaller than actinostome Alexandri. 



Test very stout and rough; v. d. .55-.65 h. d.; abactinal system 



nearly equal or even exceeds actinostome Wallisii. 



Ambulacral plates more numerous (25 or more in specimens over 40 

 mm. h. d.); valves of large tridentate pedieellariae, slender and 

 compressed. 



Pore-pairs small; poriferous areas narrow (each about one sixth 

 of ambulacrum at ambitus) with no noticeable tendency to 

 widen at peristome; arcs of pores at ambitus especially two 

 outer pairs nearly vertical ; ambulacral tubercles usually dimin- 

 ishing unequally towards abactinal system, or else unequal in 

 two halves of same ambulacrum; abactinal system usually 



large (.30 h. d. or more) with periproct about half as large . acutus var. norvegicus. 

 Pore-pairs large; poriferous areas moderate (each about one fifth 

 or more of ambulacrum at ambitus), tending to become 

 somewhat expanded at peristome; arcs of pores at ambitus, 

 distinctly oblique, becoming more or less nearly horizontal; 

 abactinal system usually about .25 h. d. with periproct not 

 usually half as large. 



Actinal membrane with scattered plates distal to the buccal 

 circle, of which some (one at least) are rounded and 

 thickened, and carry pedicellariae; valves of globif- 

 erous pedicellariae with 2-3 lateral teeth on each side. 

 Primary tubercles of ambulacra, of irregular size, not 

 diminishing equally from ambitus upward and down- 

 ward; color of test dull purplish brown . . . euryporus. 

 Primary tubercles of ambulacra, regular, diminishing 

 equally from ambitus; color variable but usually 



with some red clegans. 



Actinal membrane with numerous small thin plates, none 

 of which carry pedicellariae; valves of globiferous pedicel- 

 lariae with only one lateral tooth on each side .... lucidus. 

 Test high, v. d. two thirds h. d. or more. 



Test remarkably bare above ambitus, there being but few spines, be- 

 sides the long widely spaced primaries ailanticus. 



Test well covered with spines above ambitus. 



Test beautifully marked with deep green and white (or yellowish) ; 



primary spines very short, white (or yellowish) gracilis. 



