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 (io) and A. Diifresnii (il). 
In the above quoted work of AGASSIZ & Clark: »Hawaiian and other Pacific 
Echini. The Salenidæ, Arbaciadae . . . .» 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 »Tetrapygidæ» 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 Arbaciadæ Gray. Paît I. The Morphology of the Test in the Genera Coleopleurus 
and Arbacia. Journ. Linn. Soc. Zoology. Vol. XIX p. 53. PI. II. 6. 
