84 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



becomes rounded and flush with the adjacent plates (PL 6, figs. 9 to 

 11, and PI. 5, figs. 10 to 12), rests on an entirely diflerent footing. 

 Here the proximale is probably fused with the infrabasals, and par- 

 tially with the basals. Again, the centrale of Uintacrinus is very 

 thin, and shows no sign of a central perforation in either a dorsal or 

 ventral view. Had there ever been one, there would most certainly 

 have been a decided thickening of the plate, which, even if par- 

 tially resorbed in the adult, would be indicated in very young speci- 

 mens. Such, however, is not the case. 



Now let us turn to the third hypothesis, which is so improbable 

 on the grounds of orientation. According to Springer, in the passage 

 quoted above, a supplemental plate should conform to the angles of 

 the axial canal, being radial in a dicyclic form and interradial in a 

 monocyclic individual. This is absolutely at variance with the 

 observed condition of affairs, as is shown on Plate 9, figures 1 and 11. 



Carahocrinus, a dicyclic Inadunate from the Ordovician has a 

 decidedly pentalobate axial canal, which is radial as should be the 

 case in a dicyclic form. In a specimen from which the column had 

 been broken, careful cleaning shows that the infrabasals are distinctly 

 invaginated by the axial canal at their lower margins. It has been 

 my good fortune to obtain an adult specimen of Carahocrinus radiatus 

 from the Trenton limestone of Canada, which, probably through 

 some accident, completel}'^ lost its column during life. That this 

 separation of stem and crown occurred late in the life of the indi- 

 vidual, there can be little doubt. The stem-cicatrix on the inner 

 edges of the infrabasals is distinctly marked, and is of considerable 

 size. A comparison of the diameter of the stems of other specimens, 

 with the diameter of the cicatrix in this individual, indicates that the 

 crinoid could not have been a great deal smaller when it became free 

 than it is at present. In this specimen we find a small centrale within 

 the infrabasal circlet. In this case there was no possible chance of 

 the proximal columnal becoming transformed into a centrale. The 

 proximal columnal was fully formed, and over twdce the diameter 

 of the opening left between the infrabasals. Moreover it lay on the 

 outer faces of these plates. Such a columnal could scarcely be 

 imagined by the exercise of ever so much ingenuity to become a 

 comparatively minute plate, lying within the infrabasal circlet, and 

 abutting against the inner faces of these plates. 



Granted then that this centrale could not have been formed from 

 the proximal columnal, the question naturally arises as to what it 

 really is. The only possible answer is that it is an entirely new 

 element, functioning as a plug to stop up the axial canal. A hint as 

 to the possible derivation of this plate is given by Carpenter (1884, 

 p. 34). In Batliycrinus, Bliizocrinus, and Pentacrinus there is in 

 the lower inner portion of the dorsal cup a sort of plug formed of 



