181 
In his „Revision of the Echini“ (loc. cit.) A. Agassiz States 
to have compared specimens of A. placenta with Gray’s type- 
specimen of A. zelandiæ and found no difference between them 
which could be considered as specific. He therefore makes A. ze¬ 
landiæ simply a synonym of A. placenta and later authors 1 ) have 
followed him unhesitatingly in regarding the New Zealand form as 
identical with the Australian-Indo-Pacific form, quite overlooking 
the careful description and analysis of the two forms given by Lo¬ 
vén, by which it is proved beyond any doubt that the New Zea¬ 
land species is absolutely distinet from the Australian-Pacific form. 
How Agassiz came to the result that the characters pointed out 
by Gray as distinguishing A. zelandiæ from A. placenta were not 
valid, is hard to say. Probably he has, by some mistake, compared 
specimens only of the New Zealand species. At any rate, the Fig. 
3, PI. XIII. b. of his „Revision of the Echini“ shows conclusively 
that the species he has described and figured under the name A. 
placenta is really A. zelandiæ. Evidently the authors following 
Agassiz in regarding the two species as identical have simply 
relied on the authority of Agassiz, without examining the quest- 
ion themselves. It is especially curious that H. L. Clark, although 
he points out as an unusual zoogeographical faet that A. placenta 
occurs both at New Zealand and the Malay Peninsula (Hawaiian 
a. o. Pac. Echini. The Clypeastridæ etc. p. 43), apparently did not 
think of reexamining the question of the identity of the two forms. 
Although the characters of the two species have been very 
carefully set forth by Lov én, it may not be superfluous to point 
out here again the differences between them. 
The most conspicuous difference is that of the relative width 
of the ambulacral and interambulacral areas — the character which 
lead Gray to distinguish A. zelandiæ as a separate species. 2 ) In 
In my „Studies of the development and larval forms of Echinoderms“ 
1921. p. 96, I have also designated this species as Arachnoides placenta. 
I had at that time not had the opportunity of looking into the matter 
myself and simply followed the general use, the more confidently so 
as the identity of the New Zealand species was accepted by H. L. 
Clark in his great work on the Clypeastroids. 
G ray’s statement that in A.placenta „the outer ambulacral bands are only 
half as wide as the interambulacral ones“ evidently is a lapsus calami; 
it is the interambulacra which are mueh narrower than the ambulacra. 
