lis PROCEEDINGS OF THE \ ITIONAL MUSEUM. tol.38. 



gether, and fusing so as t<> form the so-called rosette; the stem, now 

 become too delicate to support the growing animal, breaks off at the 

 modified art iculal ion just under the nodal, and the animal becomes a 

 so-called free form. 



It is interesting to notice that in the adults of the Pentacrinitidae 

 and Comatulida the radials have undergone a considerable change 

 comparable to that undergone by the basals. Primarily large broad 

 plates forming an important part of the calyx wall, as seen in the 

 Ptilocrinida, they have gradually become recumbent, and have 

 attained an almost or quite horizontal position. Their inferior ends, 

 originally abutting on the superior ends of the basals, with which 

 they form, as in the young of Antedon, a smooth, regularly-rounded 

 cup. have, as the basals became more and more recumbent, gradually 

 slipped inward. SO as to become attached to what was at first the 

 inner side of the basals. When the radials in their turn became 

 recumbent their lower ends slipped inward along the upper (origi- 

 nally inner) surface of the basals,. so that finally the radials come to 

 form a circlet of almost or quite horizontal plates, superposed upon 

 the similar circlet of horizontal basals. The infrabasals long ago 

 underwent a similar transformation, and in the Comatulida and in 

 the pentacrinite genus Endoxocrinus have been quite lost. Morpho- 

 logically, therefore, the calyx of the Comatulida and Pentacrinitithe 

 is composed of three alternating circlets of five plates each, super- 

 posed horizontally upon each other, so that the first two have entirely 

 losl their original function of serving as a protection to the calyx con- 

 tents, and the third, or uppermost, merely forms the central part of 

 the calvx floor, having for its chief function the support of the arms. 



The changes undergone by the plates of the calyx which primarily 

 form a cup about the calyx contents are accompanied by a gradual 

 extrusion of the calyx contents, so that these (the so-called disk) 

 come to be supported more and more by the arm bases, and in the 

 Pentacrinitidae and Comatulida rest upon the post-radial ossicles toand 

 including the second of the undivided arm. In many cases these ossi- 

 cles have spread oul laterally, and are in close lateral apposition, so 

 thai they have taken on the function of the original calyx plates and 

 form a solid calcareous \\ all enclosing and protecting the "soft parts." 



Exactly the same thing has taken place in Baihycrinus, but by a 

 radically different process; instead a diminution of the interior vol- 

 ume i<\' the calyx cup by the attainment gradually of a horizontal 

 position and a slipping inward of the calyx plates, the basals and 

 radials have moved inward without at all changing their original 

 relationships, but with the same result of causing the "soft parts" 



to be extruded and to be supported by the post-radial ossicles. 



. 



ccept in Atelecrinus, where the basals are retained in the condition of those of 

 ili.' Pentacrinitidae. 



