REPORT ON THE CRINOIDEA. 167 
ermus and Symbathocrinus had a more or less rudimentary one, consisting of an 
orocentral and five oral plates only, without any radial extensions. 
The vault of the Actinocrinide is much more complex than in the Platycrinide and 
Rhodocrinide ; but its relations to the internal organs are so much better understood 
than in these families that it may well be considered first. This is more especially the 
case, as certain parts of Wachsmuth’s generalised description of the vault of these three 
families apply to the Actinocrinide only, and occasionally even to some genera only. 
It has been stated above that the vault of the Actinocrinidse formed a solid roof or 
dome over the oral surface of the visceral mass, which was covered with ambulacral and 
anambulacral plates lke the disk of a Pentacrinus. Wachsmuth has discovered that 
whatever the size and extent of the vault the apical dome plates are invariably present. 
“They consist of a central piece, occupying a position directly above the oral centre, 
which in this family is quite uniformly the centre of the disk. It is surrounded by six 
proximal plates, iterradial in position, of which four are large and equal, and two smaller. 
The four large plates are placed above the four regular interradial spaces respectively ; 
the two smaller ones, which are equivalent to and take the place of one large plate, are 
directed posteriorly, being separated from each other by anal plates or the proboscis. . . . 
There are other vault pieces occupying a radial position which are either in contact with 
those just described, or, as is more frequently the case, separated from them by a belt of 
small pieces. Their number varies considerably among species, and depends upon the 
number of primary arms, without reference to the number of bifurcations after they 
become free. They increase in proportion to the number of primary arms, in the same 
manner and on the same principle as the plates of the calyx, each order of radials has its 
corresponding plates in the vault. ... There are also interradial plates represented 
in the summit, occupying intermediate spaces between the radials, but their arrangement 
is very irregular and their number variable.”* The number of these interradial plates 
depends greatly upon the age of the individual, and is therefore very uncertain ; but as a 
general rule the number of summit plates increases regularly with that of the primary 
arms, just as that of the calyx plates does. The above statements, though true enough 
for the Actinocrinide, are scarcely so accurate as regards the Platycrinide. 
This discovery of Wachsmuth’s respecting the distribution of the radial dome plates 
in the Actinocrinidz is of enormous importance for the proper comprehension of the true 
nature of the vault of a Paleocrinoid; and, taken in connection with the embryological 
work of Goette, throws much light on the morphology of the Crinoids, and indeed of 
Echinoderms generally. 
By far the most interesting of these summit plates are the “apical dome plates” in the 
centre, which Wachsmuth says are relatively larger in young specimens. This and other 
considerations led him to point out their resemblance to the apical plates of the aboral 
1 Revision, part ii. pp. 14, 15. 
