NOTES ON ECHINODERM MOEPHOLOGY. 309 



compared to the single row of outer radials which presents 

 itself so frequently in the Crinoids. 1 



These three considerations seem to me to indicate pretty 

 clearly that it is useless to seek for the homologues of the 

 radial shields in Ophiurids among the post-radial plates of 

 Crinoids, more especially as we have not yet been able to 

 identify them in any Asterid ; and I prefer to regard them, 

 like the terminals of both Ophiurids and Asterids, as plates 

 which have no representatives in the Crinoidea. 



Fewkes also discusses the homology of those intraradial or 

 adaxial plates in the young Amphiura which I have regarded 

 as homologous with the basals of Crinoids (figs, i — in, 3) ; and 

 he suggests some doubts as to whether these plates " are basals 

 in preference to other interradial plates." 3 



The plates in question have an interradial position within 

 the ring of radials, i.e. between them and the dorsocentral ; 

 and at one stage of development they are the only adaxial 

 interradial plates (fig. in, 3). They thus correspond exactly 

 to the basals of monocyclic Crinoids, and to the so-called 

 genitals of Urchins and Asterids. In a large number of 

 Ophiurids they develop into large plates which form a closed 

 ring round the dorsocentral, with the radials immediately out- 

 side them, precisely as in the apex of an Urchin, or the calyx 

 of a stemless Crinoid like Uintacrinus. Fewkes must 

 surely be aware of this fact, for there are numerous figures il- 

 lustrating it in Lyman's report on the " Challenger " Ophiurids ; 

 and it has formed the subject of much discussion by Sladen 

 and myself. He supports his doubts by no arguments what- 

 ever, and entirely overlooks the important fact that the inter- 

 radials of a Crinoid never form a closed ring within the 

 circle of radials ; for the only inter-radially situated plates 

 which occupy such a position are the basals. It was this fact 



1 I am quite ready to admit, however, that the outer radials and the brachials 

 of a Crinoid are in a general way represented in the Ophiurid by the upper 

 arm-plates, and the row of median plates beyond the radial primaries which 

 occur in Ophiomusium, Ophioglypha, and other genera. 



2 Loc. cit., pp. 129, 146. 



