184 PROCEEDINGS OF THE ACADEMY OF [1881. 



free at this point, oi' remained included within the body walls for 

 some distance. Thej', however, characterized those plates which 

 arc immovably united with the calyx — the " distichalia " of 

 Mliller — as " pieces brachiales," and the movable joints as 

 " articles brachiaux." 



P. Herbert Carpenter, in his late work on the Actinometra, p. 

 22, states that the views of De Koninck upon the relation of these 

 plates were unquestionably correct in the case of the "Articulate 

 Crinoids," but that their application to the " fossil Tesselata " 

 was beset with some difficulties. 



We do not exactly coincide with any of these views. It seems 

 to us that either the entire radial series of plates within the calyx, 

 eventually up to the sixth division, or even higher, must be called 

 radials, or this term must be restricted to the first radial plate. 

 The first primary radials are the only plates, besides the basals, 

 which form a part of the calyx iri all Crinoids^ and which can be 

 homologized with the apical plates of other Echinoderms. All 

 succeeding plates in the series are, in our own opinion, originally 

 arm plates, wdiich by growth during the life of the individual — 

 chiefly, no doubt, in the embrj^o — and by development in geologi- 

 cal time, were enclosed within the walls, and became thus modified 

 into radials, the change being produced by growth and the devel- 

 opment of additional interradials and interaxillary pieces. That 

 this was the case in the higher orders of radials can be clearly 

 demonstrated, and we feel confident, from analogy, that the same 

 rule extends to the plates throughout the ray, which in turn 

 suggests the idea that the arms fundamentally commence with 

 the second radials, and not with the axillary plate" as intimated by 

 Carpenter, nor with the distichalia of Mliller. In practice, how- 

 ever, and in this we agree with Carpenter, it is more convenient 

 to regard the arms as commencing with the first free plate beyond 

 the calyx. 



The radials, as we designate them, consist throughout the Sphai- 

 roidocrinidffi, of five rows of plates, of two to three each, longi- 

 tudinally arranged. The upper one bifurcates and supports upon 

 its upper sloping sides two rows of one or more smaller plates — 

 the secondary radials — which in turn, either support the arms 

 directl}^, or divide again, and are followed by radials of the third 

 order. In species of the latter kind, the upper plate in one or both 

 divisions is axillary and supports on each sloping side another 



