350 HH. L. Hawkins—Ambulacral Structures in.the Holectypoida. 
as it were, include the remnants of the others; plate d will retain 
its primary character later than plate e, and when both of these 
plates have become reduced, 6 will be constantly larger than ec. The 
compound plate resulting from the maximum of crushing is composed, 
counting from its adapical suture to its adoral margin, of these three 
plates in the order J, ¢, a; 6 and ¢ being let in to the adapical portion 
of a. This feature is explained by the diagram, Fig. 1. Irregularities 
occur at times in this sequence of events, more especially near the 
peristome, but in those cases of marked departure from the scheme 
that I have noticed there is always a tendency for the regular order 
to be resumed as soon as possible. The specimens that conform to 
this outline scheme are, however, very largely in excess of those 
which exhibit modifications, and I feel confident in regarding this as 
the typical ambulacral formation among the Holectypoida. 
Fie. 1. Generalized diagram illustrating method of plate-crushing in the 
Holectypoida. 
? 
The feature of this ‘ plate-crushing’ which seems to be most 
valuable for making a study of genetic affinities is that the point at 
which the demi-plates first appear varies in a constant series. In 
a suite of twenty specimens of Conulus albogalerus, the plate of set e, 
which first becomes modified, was always No. 9 (counting from the 
latest formed plate of set a), both in the five ambulacra of each 
individual and in the whole series of specimens. ‘This fact seems to 
show that the position of the first ‘ crushing-point’ may be relied upon 
as a specific criterion. An examination of several specimens of other 
species of Conulus confirms this belief. Further generalizations must, 
however, be postponed until a critical examination of certain forms has 
been undertaken, and the resulting diagram analysed. 
The four genera here discussed are Pygaster, Holectypus, Discovdea, 
and Conulus. Species of all of these genera may be found in great 
abundance at various horizons in British Jurassic and Cretaceous rocks, 
Kaeo sesn: + 
