HOLOTHURIANS OP NEW ZEALAND. 45 



describes his Thyonidium rugosum, also from New Zealaad. 

 Of both species be remarks that they are nearly allied to 

 Thyonidium japonicmn, and yet be does not seem to bave been 

 struck witb tbeir resemblance to one another. His descriptions 

 alone, as already pointed out, are almost sufficient to establish 

 the identity of the two, there being only tv\o apparent points of 

 difference worthy of notice, the first concerning the number and 

 arrangement of the tentacles, and the second the structure of the 

 calcareous ring. 



Before proceeding to discuss these points, I should say that I 

 have examined a specimen labelled Thyone caudata, received from 

 the Dunedin Museum, and a type-slide of spicules given to me 

 by Captain Hutton and bearing the same name. 



Now as regards the number and arrangement of the tentacles, 

 I find that in the Dunedin specimen there are actually only 

 nineteen tentacles, five pairs of small alternating with four and a 

 half pairs of very much larger ones, and the smaller ones lying 

 slightly inside the larger. Thus one of the larger tentacles has 

 apparently been removed, accidentally or otherwise ; and I have 

 no doubt the arrangement was originally typical, there being 

 twenty tentacles arranged in an outer circle of five pairs of 

 larger ones and an inner circle of five alternating pairs of smaller 

 ones. I venture to think that in Theel's specimen the arrange- 

 ment of the tentacles was really the same, but that tivo of them 

 have been lost. His description strongly confirms this view of 

 the case. He says : — " The tentacles are retracted, their true 

 position being difficult to determine. In conformity witb the 

 general condition in the genus Thyonidium, the teatacles are 

 unequal and arranged in pairs, five pairs being several times 

 smaller than the eight remaining tentacles, which are distributed 

 as three pairs and two odd tentacles. Thus, these species deviate 

 from the typical forms with twenty tentacles by the circumstance 

 that two of the tentacles are unpaired." 



As regards the calcareous ring, we find that in the case of 

 "■ Thyonidium rugosum " Theel figures but does not really describe 

 it, while in the case of " T. caudatum " he describes but does not 

 figure it. It is evident, however, that the ring is practically 

 identical in- the two. In " T. caudatum " it is described as ending 

 posteriorly in "five slender bifurcate prolongations." Hutton 

 also speaks of " five bifid teeth." I must confess that these 

 descriptions alone do not convey a very clear idea to my mind. 



