280 PROCEEDINGS OP THE ACADEMY OP [1885. 



the "whole, or at least the greater part of the ventral side, is abac- 

 tinal, and this we consider one of the best distinctions between 

 the two groups. We do not understand how Carpenter can 

 maintain that those plates are orals, and at the same time can 

 retain Allagecrinus and Haplocrinus under the Palaeocrinoidea. 

 He must either refer them to the Neocrinoidea, or accept the so- 

 called " Scheitelplatten " as interradials in their simplest form. 



It seems to us that in Allagecrinus the interradials cover not 

 only the disk but also the summit plates. Culicocrinus is in a 

 similar condition, but has additional interradial plates. In the 

 somewhat higher developed Coccocrinus, the interradials are 

 separated from one another, forming lateral clefts and a central 

 gap, evidently to receive the oral plate and the ambulacra, which, 

 however, retain permanently the position which they occupied 

 before the valves separated, and rest in the bottom part of the 

 clefts. In Haplocrinus the interradials evidently separated in 

 the growing animal, and the oral plate moved outward, but not 

 sufficiently to bring it to a level with surrounding plates ; while 

 the ambulacra remained subtegminal. The interradials, instead 

 of being formed into lateral clefts as in Coccocrinus, remained 

 permanently closed by means of lateral growth, as shown by their 

 beveled edges, which are formed into grooves. 



From Haplocrinus to Cyathocrinus alutaceus and Symbatho- 

 crinus there is but one step. The latter two have proximals, the 

 former not. The proximals, we think, were introduced in the 

 Palreoerinoidea in a similar manner as the perisomic plates in 

 the Neocrinoidea. The interradials by the -increasing width of 

 the calyx retreated in the growing Crinoid toward the periphery, 

 thereby forming an open space around the oral plate which was 

 gradually tilled by the proximals and other dome plates. Sym- 

 bathocrinus is a much higher form than Haplocrinus, as shown by 

 the presence of proximals, by the ventral tube, and by the highly 

 differentiated mode of articulation. 



If it were true that the five interradials of Haplocrinus and 

 Allagecrinus are homologous with the six proximals of Symbatho- 

 crinus, Platycrinus and Actinocrinus, and that these plates are 

 orals, it would follow, inasmuch as all later and complex Palseo- 

 crinoidea have six plates, that the larger number represented the 

 higher form. And further, that Jlaplocrinus and Allagecrinus 

 had reached a degree of development such as attained only by 



