Xc 1740. TYPES OF CRINOID STEMS -CLARK. 215 



(which becomes the centro-dorsal) and the next following. Phryno- 

 crinus is the only known instance of the second case. The curious 

 fossil Platycrinus typifies the third. Among the recent forms Ihjo- 

 crinus, Ptilocrinus, Calamocrinus, and the genera of Pentacrinitidae 

 are examples of the fourth. The change from the type of stem char- 

 acteristic of the young of Antedon to that characteristic of Phryno- 

 crinus may be traced step by step in the genus Rhizocrinus, beginning 

 with R. lofotensis and ending with the gigantic R. weberi, very near 

 in stem structure, though vastly inferior in size of crown, to Phryno- 

 crinus nudus. The transition from the primitive type of stem to 

 the curiously twisted column of Platycrinus may he easily followed 

 in a good series of the young of any species of that genus, or even in 

 single specimens in which the young - stem is preserved. T have 

 observed the change from the Antedon-Wke, young stem to the radially 

 arranged adult stem in Isocrinus, and have noticed that in the largest 

 species of Bathycrinus the fulcral ridges of the articulations broaden 

 out on each side of the central canal, becoming more or less wedge 

 shaped or triangular, and breaking up into radiating lines, the articu- 

 lations thus approaching the uniformly radiated type found in 

 Calamocrinus and Ptilocrinus so closely as to leave no possible doubt 

 as to their mode of origin. 



It might be urged that the articular faces of the columnars of the 

 Pentacrinitidae, with their petaloid markings, could not be placed in 

 the same class with articulations like those of Calamocrinus, where 

 the joint faces are uniformly marked with radiating lines; but in the 

 Pentacrinitidae it is merely a case of the columnars, primarily with 

 articular faces bearing regular radiating lines, being moulded or cast 

 into petaloid sectors by the under surface of the basals against which 

 they are formed, these basals being in a curiously reduced condition, 

 midway between the normal type of basal, as seen in Calamocrinus or 

 Ptilocrinus, and the atrophied and metamorphosed condition seen in 

 Antedon. a 



I can see no other way of deriving the stems of the recent and 

 most fossil crinoids than by supposing them to be the homologue 

 of the central plate of the crinoid-echinoid ancestor which has gradu- 

 ally become thickened and elongated and developed transverse 

 alternating fractures which have metamorphosed into definite artic- 

 ulations. The fact that, when viewed by polarized light, the axis 

 of crystallization is seen to follow the axis of the stem while in the 

 basals and radials it passes at right angles to the plane of their 

 surfaces, and therefore also in the same direction toward the center of 

 the calyx would seem to suggest thai the sum of the columnars was 



"Since the above was put in type there has come to lighl a remarkable genus, 

 Pro isocrinus, in which the lower pari of the Btem resembles thai of Calamocrinus, but 

 the upper thai of Isocrinus showing thai this transition, foretold by deduction, actu- 

 ally occurs. 



