183 



p. HERBERT CARPENTER. 



a very early stage of development, simultaneously with those 

 of the other plates of the skeleton, and (as I believe) carry 

 away the central abactinal plate from the neighbourhood of 

 the basals. A suggestion essentially similar to the one just 

 made has already been put forward by Wachsmuth/ who 

 regards the central abactinal plate as the basis, a view 

 which, as I have already endeavoured to show, is no longer 

 tenable.2 Wachsmuth also compares the interradial peri- 



pheral plates (orals) of the actinal surface to the so-called 

 parabasals or subradials of the abactinal system, which 

 '^ were undoubtedly the Jirst developed parts of the dorsal 

 side, and the parts which are the most highly developed in 

 the Cystideans." It will be noted how this passage 

 strengthens the arguments which I have already brought 

 forward in favour of the view that the so-called subradials 

 are the true basals of those Crinoids in which they occur. 



The diameter of the visceral mass of the Pentacrinoid is 

 so slight that the orals are sufficient to cover it in com- 

 pletely on the ventral side. But in the P alaocrinoidea it is 

 not only absolutely, but also relatively, much larger, so that 

 the oral or '^ apical " system alone is insufficient to 

 cover it. 



Accordingly we find that the ^' apical" plates do not make 

 up the whole of the vault of the Palaocrinoidea, but " there 

 are other summit plates following a radial direction, which 

 are either attached to the apical pieces or separated from 

 them by a belt of small polygonal plates," Avhile the inter- 

 mediate spaces between these radial areas are occupied by 

 the interradial plates of the vault. The number and arrange- 

 ment of these plates vary greatly in different species, accord- 



1 Loc. cit., p. 189. 



' Part T, this Journal, Vol. XVIII, pp. 360, 370, 371. 



