308 P. HERBERT CARPENTER. 



the apical system, Fewkes's views are those of his predecessors, 

 as indeed he himself admits. But he adds a suggestion which 

 he considers as a direct sequence of the homology of the 

 primary radials in Ophiurids and Crinoids, respectively. 1 It is 

 this: " The homology of the radial shields of the Amphiura 

 with the first brachials of the Crinoid would seem not unrea- 

 sonable. The only paired plates of the arms with which they 

 could be compared are the adambulacral. With these, however, 

 they have little resemblance save in their double origin/' If, 

 as I suppose, Fewkes uses the term " first brachials " to denote 

 the lowest joints of the two arms which are borne upon the 

 radial axillaries of Pentacrinus, Ante don, and most recent 

 Crinoids, there are at least three objections to an homology 

 between them and the radial shields of the Ophiurids. 



1. Many Crinoids, e.g. Hyocrinus, Rhizocrinus, 

 Eudiocrinus, Cupressocrinus, &c, have no paired first 

 brachials at all, for there are only five arms, one on each 

 primary radial. 



2. In those Crinoids which have ten or more arms there may 

 be from one to seven radial plates between the primary or calyx- 

 radials and the paired first brachials. The only genera in 

 which the latter plates ever rest directly on the primary radials 

 are the aberrant Allagecrinus and Tribrachiocrinus. 

 But this is' not the case all round the cup, so that there are 

 never ten first brachials to correspond to the ten radial shields 

 which are so constant in Ophiurids. Then, again, each of the 

 two larger radials in Catillocrinus may support as many 

 as thirty arms. Where are the homologues of the two radial 

 shields among all these first brachials ? 



3. The radial shields of Ophiurids are invariably present, but 

 they have no permanent relations to the primary radials. It is 

 not unlikely that they are always developed immediately outside 

 the primaries, where they remain in many species. But, on 

 the other hand, they are often separated from the primaries by 

 a series of intermediate plates which exhibit no general 

 constancy of arrangement, and cannot therefore be directly 



1 Loc. cit., p. 130. 



