Bd. VI: 4) THE ECHINOIDEA. 35 



It will doubtless be agreed that the characters here pointed out, especially the nume- 

 rous abactinal interambulacral tubercles and spines, the robustness of the spines and 

 the shape of the ocular plates, make this a very well characterized species. It seems to be 

 nearest related to Dufresnii (the green colour, the lack of spicules); the close tubercula- 

 tion reminds one of A. lixula, but with this species it is scarcely more nearly related. 



Though I have not seen any of the other specimens from Nightingale Island, 

 I think it will not be too hardy to suggest that they also belong to this species. 

 This locality is then to be removed from Dufresnii, which is known with certainty 

 alone from the South American Coasts, as explained above (p. 31) and from the 

 Antarctic Coast opposite the extremity of South America. 



Figs. 10 — 11. Transverse sections of spines of. Arbacia crassispina (10) and A. Dufresnii (11). 



In the above quoted work of AGASSIZ & CLARK: .Hawaiian and other Pacific 

 Echini. The Salenida;, Arbaciadas ....■» it is stated (p. 109, Note) that in my dis- 

 cussion of the systematic value of the character afforded by the ambulacral structure 

 of the regular Echinoids (Siam-Echinoidea I. p. 42), the result being that the three 

 main types of ambulacral structure, viz. the cidaroid, the diadematoid and the echi- 

 noid type, must be regarded as characters of orders, I have made a curious slip, 

 having overlooked that DUNCAN & SLADEN have shown the ambulacra of Tetra- 

 pygus to be of the echinoid structure, not of the diadematoid, as are those of the 

 other Arbaciids. They conclude the note with the remark: »Are we to presume that 

 Dr. Mortensen will establish a family »Tetrapygidae» under his »Tribus 4. Echinina* 

 for this aberrant genus ?» 



It is true that I have overlooked DUNCAN & SLADEN's statement * about the 

 ambulacra of Tetrapygus, and I have no excuse for that. But I might suggest 



* On the Family Arbaciadoe Gray. Part I. The Morphology of the Test in the Genera Coleopleurus 

 and Arbacia. Journ. Linn. Soc. Zoology. Vol. XIX p. 53. PI. II. 6. 



I 



