88 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



regard to the relative size of the centrale. We find in Uintacrinus 

 that the centrale varies considerably in size, apparently irrespective 

 of whether it occurs in Form M or D, or of the size of the individual 

 to which it belongs. Apparently, however, the centrale in Form D 

 is "often very small," In some cases where fusion does not take 

 place the centrale increases somewhat in size. There is indeed no 

 good reason why the centrale in Uintacrinus should not show varia- 

 tion in size in different individuals. In Marsupites we find that the 

 central apical plate, wliich in the young must have been quite small, 

 became relatively enormous in size. The increase in size of the cen- 

 trale in Uintacrinus is similar in kind, if not in degree, to that shown 

 by Marsupites. The variation in size of the centrale in Form D may 

 readily be explained. If the fusion of the infrabasals with the cen- 

 trale took place early in the life of the individual, a small plate would 

 have been formed of necessity. This might or might not have in- 

 creased in size during the subsequent growth of the animal, the size 

 being again merely a matter of individual variation. If the infra- 

 basals fused with the centrale later in the life of the crinoid, as in 

 some cases they certainly did, the resultant plate would have been 

 of comparatively large size. This plate might still be added to, 

 making it even larger. It seems to me that the foregomg arguments 

 explain the variations in the size of the centrale in Forms M and D, 

 and the occurrence of both types of structure in young and adult 

 individuals, features upon which Springer lays particular stress on 

 page 34 and elsewhere. 



In regard to the possession of the two types of base by Uintacrinus, 

 Springer, on page 25, says: "It is not a case where the infrabasals 

 are concealed, or are more or less perfectly or imperfectly developed, 

 or are used with some other element, as in some forms which Bather 

 calls pseudomonocyclic. For here Wachsmuth and Springer's law 

 of alternate arrangement of the elements of the Crinoid skeleton 

 strictly prevails." As I have previously shown, the orientation of 

 the centrale is of little or no importance, being dependent entirely 

 upon the character of the surrounding plates. In Form D the cen- 

 trale fits into the angles between the infrabasals, and is, therefore, 

 interradial. In Form M the centrale (consisting now of the original 

 centrale, to wliich the infrabasals have fused) adjusts itself to the 

 angles of the basals, and is, therefore, radial. Springer also uses 

 this argument against the possiblity of the fusion of infrabasals and 

 centrale at the bottom of page 30. It is curious to note in this con- 

 nection, that on page 32 in considering the possibility of a fused 

 condition of centrale and infrabasals he says: "A coalescence or 

 fusion of the infrabasals with the centrale would not be subject to 

 the same objection on the ground of orientation. The resulting 

 plate would be radial, as it should be in a monocyclic form." 



