ARBACIA PUNCTULATA. 263 



ARBACIADAE. 



Family Arbaciadae, Gray, 1855. Proc. Zool. Soc. Loudon. 



ARBACIA. 



Arbacia Gray, 1835. Proc. Zool. Soc. London. 



Test thick, moderately large, subcorneal, actinal surface flattened. Ambu- 

 lacral areas narrow, poriferous zone exceedingly narrow above ambitus, com- 

 posed of simple vertical pairs, but becomes greatly petaloid owing to flattening 

 of test near actinostome. Tubercles of uniform size, neither perforated nor 

 crenulated, forming two irregular vertical rows, in the ambulacral zone, and 

 from four to twelve in the interambulacra, arranged at same time in more 

 or less regular horizontal and vertical rows. Actinostome large, no deep 

 cuts, but very prominent ambulacral lips. Auricles disconnected, teeth like 

 those of Diadematidae and Cidaridae. Anal system composed of four large 

 plates. The bare space of median interambulacral space is due to the resorp- 

 tion of the large tubercles, changed into a coarse granulation which is crowded 

 with pedicellaria along the ambulacral area. Spines stout, long, often spath- 

 iform, having a peculiar internal structure partly resembling that of Cida- 

 ridae. Buccal membrane bare. 



Arbacia punctulata 



! Echinus punctulatus Lamk. 1816. Ann. s. Vert. 



! Arbacia punctulata Gray, 1835. Proc. Zool. Soc. London. 



PL II f. 4; PI V.f.i-is. 



The species of Arbacia have been too much multiplied, owing to the pre- 

 dominance given to characters which are now known to be either due to 

 differences of age or to very variable features. The character of the abac- 

 tinal system, the number of coronal plates, with the mode of arrange- 

 ment of the interambulacral tubercles, as well as the proportions of the 

 actinostome, furnish us with characters by which we can readily distin- 

 guish the species enumerated in this Revision. The two species found 

 in the Atlantic are easily distinguished by the great difference in size of the 

 abactinal system, the genital plates forming in A. punctulata a large promi- 

 nent pentagon ; the ocular plates are small and frequently reach the anal 

 system, though usually they are excluded from it ; the coronal plates are 



