NO. 1846. ON CERTAIN ELEUTHEROZOIC PEL-MATOZOA—EIRK. 85 



calcareous matter. In these forms, this phig only extends to the tops 

 of the basals, and is of rather loose texture. It shows, however, the 

 tendency of calcareous matter to be deposited in the basal portion of 

 the cup, which, in case of the detachment of the crinoid from its 

 column, could readily be formed into a solid plug. A relatively 

 slight extension of this calcite-depositing tissue would completely 

 fill the axial canal, and form a plate which on the exterior would be 

 flush with the plates of the proximal circlet. Thus it is probable that 

 in the case of Carahocrinus at least, the centrale was formed from 

 within, outward. 



The most astonishing thing about this plate in Carahocrinus is that, 

 contrary to all expectations, it does not form a stellate plug which 

 merely fills up the axial canal, and the angles between the infra- 

 basals, but actually constitutes a pentagonal interradial centrale. 

 The orientation of a central, apical plate, then, if it lies withm the 

 proximal circlet of plates, and ''abuts against them by their inner 

 faces" is of absolutely no importance. The centrale in all cases 

 conforms to the angles of the adjacent plates and not to the lobes 

 of the axial canal. Another instance of a plate conforming to the 

 surrounding plates has already been given in the case of Antedon. 

 Here we find in the early stages that the proximale is pentagonal, 

 and radial in position, its shape being controlled by the basals. 

 Later, when the centrodorsal has overgrown the basals, the shape 

 of the plate is governed by the general contour of the calyx and the 

 presence of cirri. These factors are of course, not at all of the same 

 nature as the one that controls the plate when it lies within the 

 proximal circlet. In the latter case it is the absolute lateral con- 

 tact of the adjacent plates that governs its form. 



The centrale of Uintacrinus, I take it, was an entirely new element, 

 formed much in the same manner as the plate in Carahocrinus. The 

 presence of infrabasals and the centrale in some of the individuals 

 precludes the possibility of its having been formed by the fusion of 

 the infrabasals. 



Significance of hoih mono and dicyclic hases in Uintacrinus. — The 

 presence in Uintacrinus of apparently both dicycHc and monocyclic 

 types has been considered as of the utmost importance by Springer 

 and others, as conclusively showing the unimportance of the features 

 upon which Bather has made his main divisions of the Crinoidea. 

 Mr. Springer's views on the subject may best be explained by a 

 quotation from his work on Uintacrinus (1901, p. 30): 



If the two forms of base represented by text figures 1 and 2 had been found in speci- 

 mens otherwise separable, they would, under Mr. Bather's arrangement, have been 

 unquestionably referred to different genera, families, orders, and subclasses. Con- 

 sidering the apparent identity of these forms in every other point of structure, coupled 

 with their mode of occun-ence and association, I do not see how any such separation 

 can possibly be made in this case. We therefore have apparently to deal with a case 



