424 COLOBOCENTROTUS ATRATUS. 



pentagonal, large, sensibly indented. Auricles very slender, connected by 

 low ridges. Tbe whole upper surface and the greater part of test to 

 ambitus is covered with short spines forming a close cuirass of polygonal 

 blocks; the spines of the ambitus and lower surface are more or less cylin- 

 drical, but short and frequently spoon-shaped. Buccal membrane bare. 

 .laws remarkable for tbe large deep open foramen extending far down 

 towards tbe teetli on tbe face of the pyramid. 



Colobocentrotus atratus 



Eckinus atratus Linn - .. 1 75S, Svst. Nat. 

 Colobocentrotus atratus Br., '835, Prod. Des. An. 



PL III d . f. s; PL XXX VI. f. e, r; PL XXXVIII. f. n, 12. 



Tbe whole abactinal part of the test above the ambitus is covered by a pave- 

 ment of closely packed, irregularly shaped short hexagonal spines, scarcely 

 increasing in size towards the ambitus. At ambitus the spines lengthen 

 somewhat, become slightly spathiform or cylindrical, often club-shaped, and 

 form a more or less regular circle of prominent spines round the edge of 

 the test, carried by from two to three rows of primary tubercles near the 

 ambitus. On the actinal surface the spines are short, stout, cylindrical, 

 resembling short spines of Echinometra. The abactinal system is solid, 

 compact, covered by large secondary tubercles carrying spines similar 

 to those of the rest of the test, so that, when covered with spines, the 

 abactinal system is completely concealed. The anal system is small, ellip- 

 tical. Madreporic genital large, prominent^ greatly exceeding in size the 

 other genital plates. Genital plates pentagonal; ocular plates triangular, 

 excluded from the anal system, and not projecting beyond the circular line 

 of the abactinal system. The poriferous zone is broad ; the arcs of pores 

 composed of from eight to twelve pairs of small pores. The arrange- 

 ment of these arcs is quite irregular, the vertical crowding of the coronal 

 plates during their growth being so great that in older specimens it becomes 

 nearly impossible to trace the pores which belong to the same arc of the 

 poriferous zone ; the whole poriferous zone being covered by short discon- 

 nected arcs, irregularly scattered, without any definite arrangement when 

 seen from the outside. 



From the ambitus the tubercles of the ambulacra] zone diminish gradually 

 in size towards the abactinal pole, and rapidly towards the actinostomc ; 

 they are arranged in two wvy regular rows, separated by a median line cf 



