162 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



the centrodorsal separates from the radials more readily than they do from each 

 other. 



The fusion between the infrabasals and the centrodorsal in the comatulids 

 has never been studied, but apparently the coalescence is complete, for the centro- 

 dorsal of the adults never shows any evidence of being other than a single element. 



From what we know of the stalked crinoids we are safe in saying that in 

 almost all cases the calyx plates are united in the same manner as the radials of 

 Endoxocrinus. But all stages may be traced between the purely ligamentous 

 union and a soldering by calcareous deposit so close that no suture lines are 

 discoverable. 



The development of the synostosis or anchylosis is probably due to a gradually 

 increasing continuity of the mesodermic lime depositing layers in which the ossicles 

 are formed. Normally there is complete discontinuity between the sectors includ- 

 ing each ossicle ; with the less and less complete differentiation of this mesodermic 

 layer into definite sectors the amount of deposition of calcareous matter between 

 the ossicles increases until in those forms, as the species of Ilycr-inus, Bathi/crinus, 

 and Rhisocrinus, and certain species of Ptilocrinus, in which the mesodermic layer 

 over the basal ring is practically uninterrupted, an almost uniformly continuous 

 basal ring is formed in which there is no trace whatever of interbasal sutures. 

 Carried still farther, the continuity of the mesodermic layer becomes complete, and 

 thus a single plate which is the equivalent of two primitive plates is formed, as 

 in the case of the large basal in Hyocrinus, Thalassocrinus, and Gephyrocrinus. 



NON-MUSCLT.AR UNIONS. 



General remarks. 



All the nonmuscular unions between adjacent ossicles in the crinoids, whether 

 they occur between the plates of the calyx, the radials, the columnals, the cirrals, 

 the brachials, or the pinnulars, are fundamentally identical in nature, and so far 

 as they have been studied they have been shown to be histologically the same. 



In many fossil types little or no differentiation of these unions according to 

 location can be detected, but in the elongated and slender appendages of the 

 comatulids they ordinarily occur in two distinct forms, one in which the apposed 

 joint faces are marked with numerous radiating ridges, and the other in which they 

 are traversed by a median ridge separating two ligament masses of usually equal 

 size, as in the synarthries between the paired ossicles in the division series and in 

 the arm bases and in the articulations between the pinnulars, cirrals, and larval 

 columnals. 



In the pentacrinites, which with the comatulids form a single well circum- 

 scribed group, the apposed faces of two ossicles united by syzygy, both in the 

 arms and in the column, are smooth and unmodified, like the apposed surfaces of 

 two adjacent radials. This indicates that the radiating ridges characteristic of 

 the syzygial faces of the brachials in the arms of the comatulids can not be of any 

 particular phylogenetic significance. 



