652 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 40. 



horizontal underbasals, so that the} 1 " eventually form a horizontal 

 circlet of plates superimposed upon the similar circlet composed of 

 underbasals; this condition is seen in Antedon just before the trans- 

 formation of the basals into the rosette. A somewhat similar state 

 of affairs is seen in the pentacrinites, though here the basals are not 

 quite so reduced and, instead of covering the underbasals, as in 

 Antedon, they imbricate over them. The radials travel the same 

 path as did the basals and underbasals before them, and in most of 

 the comatulids have become quite horizontal, serving merely as a 

 platform upon which the visceral mass rests, and for the attachment 

 of the arms. 



This developmental path is very plain and easily demonstrated; 

 but in Antedon the underbasals at their first appearance are very 

 small and irregular in number, while in Cornactinia, Comanihus, 

 Hathrometra, and Cornpsometra, according to the investigations of 

 Mortensen and myself, no underbasals ever appear. It seems clear 

 that in the recent comatulids acceleration of development has 

 operated to push the metamorphosis of the underbasal circlet so far 

 forward in the ontogeny that it either only appears as a transient, 

 usually imperfect, circlet or not at all; from what we know of the 

 transformation of the basals and of the radials we must assume that 

 the underbasals were of equal importance. This reasoning demon- 

 strates that there is no tangible difference between the calyx of 

 Marsupites and that of a comatulid in the younger stages, excepting 

 only for the occurrence of a central plate, end to end with the under- 

 basals, in the former. 



The central plate in Marsupites, like the similar central plate in 

 Vintacrinus , I believe to be the homologue of the dorso-central 

 (terminal stem plate) plus all the columnars of the comatulid. In a 

 previous paper I traced out the development of the crinoid stem from 

 a hypothetical primitive central plate such as is seen in certain 

 echinoids. I assumed that the central plate in Marsupites and in 

 Vintacrinus was a primitive central plate, and in no way comparable 

 to the comatulid centrodorsal (the topmost columnar of a subse- 

 quently discarded stem) for the following reasons: It lies in the body 

 wall flush with the surrounding underbasals, and therefore can not 

 be a columnar, for in all stalked crinoids the topmost columnar sup- 

 ports more or less of the lower margin of the basals or of the under- 

 basals; this is a mechanical necessity, as otherwise the weight of all 

 the calcareous structures would have to be taken up by the soft 

 interior structures immediately above the stem and by the sutures 

 between the topmost columnar and the underbasals. As the under- 

 basals of the young Antedon surround its apical system in just the 

 way that the underbasals of Marsupites and of Vintacrinus surround 

 their central plates, I see no escape from the conclusion that these 



