REPORT ON THE CRINOIDEA. 167 



crinns and Symhathocriniis had a more or less rudimentary one, consisting of an 

 orocentral and five oral plates only, without any radial extensions. 



The vault of the Actinocrinidae is much more complex than in the Platycrinidse and 

 Ehodocrinidje ; 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 Actinocrinidse 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 like the disk of a Pentacnniis. 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 rmiformly the centre of the disk. It is surrounded by six 

 proximal plates, interradial 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 then- 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 Actinocrinidse, are scarcely so accurate as regards the Platycrinidse. 



This discovery of Wachsmuth's respecting the distribution of the radial dome plates 

 in the Actinocrinidse is of enormous importance for the proper comprehension of the true 

 nature of the vault of a Palaeocrinoid ; 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. 



