NO. 1846. 0\ CERTAIN ELEUTHEROZOIC PELM4.T0Z0A—KIRK. 83 



* * * it can not be (1) the fused infrabasals, because it is found here coexistent 

 with them, a distinct element, situated within their circlet. There are thus left the 

 other possibilities, viz: (2) That it is the representative of the proximal or distal 

 stem ossicle; or (3) "a simple supplementary plate developed to fill up the gap left 

 by the disappearance of the stem." 



To the second and third hypotheses he objects (p. 28) on the 

 ground — 



That the centrale is located within the circlet of infrabasals, and abuts against 

 them by their inner faces, instead of by their outer or dorsal sides. It does not 

 envelop or conceal the infrabasals, as the proximal columnal does in pseudo-mono- 

 cyclic forms; nor the basals and radials, as the representative of the stem in the 

 Comatulse usually does, although there is an exception to that in the case of certain 

 living species of Actinometra, and in many fossil Comatulse. On the other hand, the 

 orientation of the centrale is precisely as the stem should be; i. e., interradial when the 

 infrabasals are present, and radial when they are not. And the orientation is strongly 

 against the third of the above suppositions. For the gap left by the disappearance 

 of the stem, and to be filled up by stereom, would be the axial canal piercing 

 the base of the calyx at the center of the basal or infrabasal ring. This would be 

 radial in the latter case and interradial in the former. A supplemental plate 

 developed to fill up this space should have the same orientation; but this is just the 

 reverse of what is exhibited by the centrale. It seems to me, therefore, that the 

 argimient is decidedly in favor of the view that the centrale is a relic of the stem of 

 the Stalked Crinoids, if not, indeed, of the pedunculate stage of Uintacrinus itself. 



This last statement is explained more at length on page 59, where 

 he implies that the centrale is a modified proximale as in Millericrinus 

 prattii, Actinometra, and other Comatulaj. 



Probable origin of the centrale. — At this point I wish merely to con- 

 sider the second and third possibilities given by Bather, and the 

 objections raised in regard to them by Springer. Against the hypoth- 

 esis that the plate represents the proximal or distal stem ossicle, he 

 brings the argument that the centrale lies within the circlet of infra- 

 basals, and "abuts against them by their inner faces, instead of by 

 their outer or dorsal sides, etc." This objection I consider insuper- 

 able, although Springer, as above quoted, apparently does not, con- 

 sidering such a solution of the problem the most reasonable one. In 

 no case do we know of the proximal columnal taking part in the 

 dorsal cup except as a proximale, fusing mth the infrabasals which 

 it overlies. Nor can we readily imagine a proximal columnal, resting 

 on the outer faces of the infrabasals, consolidated into a plate which 

 is squeezed into an opening of the size of the axial canal with which, 

 as a columnal, it was itself perforated. 



An obvious argument against tliis last conception is that the sepa- 

 ration of the crown and stem took place very early in the ontogeny 

 of the animal, and that the proximale, now the centrale, enlarged 

 but very slightly or not at all in the subsequent growth of the crinoid. 

 Even at this early stage, however, the proximale would have had to 

 fit into an opening smaller than itself. The cases among Actinometra 

 where the cirri are lost, and the centrodorsal by partial resorption 



