20 BULLETIN OF THE 



Diadema mexicanum A. Ag. 



Abactinal system much smaller in proportion to actinal than in any other 

 species of the genus. Spines exceedingly long, equalling in length twice 

 the diameter of test, moderately stout. Outline of spherosome perfectly 

 circular, regularly arched in profile. Cuts of actinal system slight. The 

 large tubercles extend almost to abactinal system. — Acapulco. 



Diadema globulosum A. Ag. 



This is a small species, perfectly globular, with only four rows of large tu- 

 bercles in interambulaerum ; abactinal system depressed. Remarkable for 

 the great length and extreme slenderness of the spines ; they are at least 

 three times the diameter of test; actinal portion of test very convex. — 

 Kingsmills and Society Islands. 



Echinocidaris Desml. Etud. Echin. (emend.). — Agaritcs Ag. Cat. Rais. 



Eehinocidaris punctulata Dbsml. Etud. Echin. 



Syn. Echinocidaris (Agarites) punctulata Ac Cat. Eais. — Charleston, 

 South Carolina. 



Echinocidaris Davisii Ao. 



Differs from the South Carolina species in having a greater number of 

 tubercles closely packed together. Spines quite short, granulation round 

 the primary tubercles very prominent. Color of test and spines dark vio- 

 let, almost black. Tubercles very crowded in ambulacral space. — Naushon, 

 Massachusetts, south of Cape Cod. 



Echinocidaris incisa A. Ag. 



Abactinal system very prominent, sutures between the plates well marked ; 

 tubercles large, spines short, stout, color yellowish-brown. — Guayamas, 

 Panama. 



Arbacia Grat (non Ac). — I'drapjgus Ag. Cat. Rais. 



A.rbacia nigra Geay. 



Syn. Echinocidaris (Tetrapygus) nigra Ag. Cat. Rais. — Mejillones. 



Arbacia asquituberculata Gray. 



Syx. Echinocidaris (Tetrapygus) aquituherculalu Ag. Cat. Rais. — Fayal. 



Echinostrephus A. Ag. 

 Small sea-urchins with tubercles resembling those of Holopneustes in 

 their arrangement, witli narrow poriferous zones, pores arranged in arcs. 

 Abactinal system raised above level of abactinal part of test. Large geni- 

 tal plates occupying nearly the whole of this system. Actinal system large, 

 circular, no indentations. Spines long, slender, longitudinally striated. 

 Test convex near actinal portion, flattened above, the greatest diameter 

 being nearer the abactinal pole. Auricles of medium size, with a large 

 opening and no connecting ridge. Teeth provided with transverse arc. 



