30 SMITHSONIAN INSTITUTION 



where the infrabasal circlet is much smaller in area than the proximal columnal, 

 and takes the form of a plug at the center of the basals, in which the plates ' 

 stand at a different angle from the stem ossicles (text-fig. n). 



It now seems clear that the idea of fusion between infrabasals and top 

 columnal so as to prevent the interpolation of new segments must be limited to 

 those crinoids in which the stem is only a temporary structure, and the infra- 

 basals disappear at an early stage by absorption into the proximale which takes 

 the place of a stem, or in which the infrabasals are so enveloped by the 

 proximale as to lose their identity completely. Therefore it only applies to the 

 comatulids and some Mesozoic crinoids, and not to the Flexibilia as now re- 

 stricted. W. B. Carpenter's luminous description x of the development of the 

 stem in the larval Antedon rosaceus, in which he shows that while the original 

 segments are advancing toward completion " new segments are being devel- 

 oped .... between the highest of these and the base of the calyx," took no 

 account of the infrabasals, whose existence in the comatulids was at that time 

 unknown. In the closely related species in which these plates were first dis- 

 covered they are fused with the top columnal at a very early period in stem 

 growth; while in Promachocrinas, in which they are better developed than in 

 any other known comatulid, they are fused with the upper ossicle in the pre- 

 brachial stage, and disappear from view externally long before the stem is 

 completed and cast off. 



Mr. Austin H. Clark has given much attention to the study of the column, 



which he finds in the Mesozoic and later crinoids to present the most reliable 



characters for broad systematic differentiation, in contrast to the Paleozoic 



forms in which the variations in calyx structure far outweigh the characters 



offered by the column. In his Monograph of the Existing Crinoids, 2 referring 



to the proximale in Recent and Mesozoic crinoids, and having explained that 



proximales, or columnals homologous to proximales, are always attached to the 



columnals just below them by a close suture called the stem-syzygy, and that 



the nodals throughout the stems in which they occur, with their syzygially 



united infranodals, are reduplications of the proximale, he states on p. 213: 



The formation of the proximale, closely attached to the dorsal surface of the calyx and 

 fused with the infrabasals, prevents the formation of new columnals above it, and marks the 

 maturity or end of growth of the stem. But the columnal formation may continue by inter- 

 calation between the columnals immediately below the stem syzygy, or excessive vegetative 

 power may shove the proximale outward before it fuses with the calyx. In the adult penta- 

 crinites new proximales are continually forming beneath the calyx, where every new columnal 

 formed is a proximale, only to be pushed outward by younger ones. Later these become 

 separated by intercalated segments, each of them becoming united by syzygy to the inter- 

 calated segment immediately below it. 



1 Philosophical Transactions, Roy. Soc, London, 1866, sec. 63, p. 730. 



2 Bull. 82, U. S. National Museum, 1915, p. 212 et seq. 



