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THE AMERICAN NATURALIST [Vol. XLIII 



an adaptation to special conditions of existence. The 

 change from a biserial to a monoserial arrangement in 

 the crinoids is merely a matter of an elongation of the 

 arm, and a slipping in of the joints in one series between 

 those of the other (Fig. 9). It has been suggested that 

 Encrimts, Platycrinns and the other biserial crinoid 

 genera are derived from monoserial ancestors because the 

 new joints as they are formed at the arm tip are always 

 monoserial in arrangement; but apart from the mechan- 

 ical difficulties in the way of biserial termination to a 

 free arm like that of the crinoids, we find that almost the 

 same is true in the urchins ; plates are added one by one 

 abutting upon the median ambulacral line just behind the 

 oculars, which move out by lateral growth first to one side 

 then to the other, just as the monoserial plates at the tip 

 of a Platycrinus or Encrimts arm, on increasing in size, 

 more laterally first to one series, then to the other; the 

 supposedly monoserial tip in biserial crinoid genera, 

 therefore, appears to be, in reality not monoserial at all, 

 any more than the proximal (post ocular) portion of the 

 ambulacra in the urchins is monoserial. 



The slipping in of the two series of joints in the 

 biserial crinoid arm in the transition from a biserial to a 

 monoserial condition undoubtedly first gave rise to syn- 

 arthries and syzygies, the former originating from the 

 coalescence of two straight muscular articulations, the 

 latter from the coalescence of two oblique muscular 

 articulations. I have been unable to study the joint faces 

 of crinoids of the biserial type, and therefore have not 

 traced the process, but, judging from the data at hand 

 there seems to be much in favor of such an origin for 

 spnarthries and for syzygies, and I hope to be able to 

 adduce additional evidence in support of it in the future. 

 A study of the ontogeny of Antedon does not help us, 

 for in that type the synarthry and the syzygy are onto- 

 genetically older than the beginnings of the deposition 

 of calcareous matter, and the synarthry between the first 

 two postradial joints is ontologically nearly or quite as 



