28 
I cannot afford the necessary time, for the present at least. The 
last of the quoted papers, however, contains a passage which has 
made me think it appropriate to take up a single point for treat- 
ment at present, the more so as this is one of the main points 
in the question about the relationship between Crinoids and Echi- 
noids, viz. the central (suranal) plate of the Echinoids. 
In the quoted paper ,,0n the Origin of certain types of Cri- 
noid stems" Dr. Clark says (p. 211) that I have criticized him 
for homologizing the column of the crinoid with the sur-anal plate 
of the urchins ,,0n the ground that the so-called ,,Palæoechinoidea”, 
the oldest known echinoids, lack the sur-anal plate". It might 
appear from this that the lacking of the central plate in the 
Palæechinoids was the only reason which I could produce against 
homologizing the central plate of Echinoids with the stem (or the 
dorso-central) of Crinoids. This is, however, very far from being 
the case; the Palæechinoids afford - only one of several reasons 
against that homology; I may be blamed for having named only 
this single point in my letter, but I may give the excuse that 
it was a letter, not a scientific paper. — Thinking now (partly 
through my fault) that the Palæechinoidea are the only forms of 
Echinoids in which the central plate is lacking, Clark has done 
away with this difficulty for the said homology through the follow- 
ing reasoning: ,,In the structure of the test the ,,Palæoechinoidea" 
are in certain ways far more specialized than any recent species, 
and, as specialization is usually accompanied to a greater or lesser 
degree with the suppression of more or less fundamental primitive 
structures” it may be understood that the central plate has been 
suppressed here. ,,I (Clark) assumed that, although te sur-anal 
plate was usually retained in a more or less reduced form by 
all recent types"), there was no reason for supposing that, were 
the recent genera to attain multicolumnar ambulacral and interam- 
bulacral areas instead of their more primitive bicolumnar areas, 
1) The italics are mine. 
