•380 I'ROCEEDINCiS OF THE ACADEMY OF [1890. 



only changing its form enough to fit in between adjoining plates. 

 Later on, as the ventral sac grew still larger, and the brachials came 

 to occupy the entire upper face of the radials, leaving no surfaces for 

 an attachment of anal plates, the right ])osterior radial was gradu- 

 ally shifted to the I'ight, and finally attained a position directly 

 above the right postero-lateral basal, and obliquely above the radi- 

 anal. Toward the end of the Carboniferous, when the size of the 

 anal tube was again reduced to a minimum, both anal plates grad- 

 ually disappeared, and the five radials resumed their normal position. 



Mr. Bather assumes, as before stated, that the anal plate, the 

 plate X, is derived primitively from a brachial, which gradually in 

 geological times passed downward into the dorsal cup. He 

 regards (p. 329) "those forms as px'imitive in which the radianal is 

 more of a radial and less of an anal, in which it is not in an asym- 

 metrical position but corresponds to the other lower radial plates. 

 Such forms are locrinus, Heterocrinus, Ectenocrinus, Anomalocrinus 

 and Merocrimis. Now in all these forms X is supported by E, and 

 does not touch R'. Obviously then X is not derived from R', but orig- 

 inates above R, and on its left side. By parity of reasoning we assume 

 the next stage to be represented in such forms as Hybocrinus (?), 

 Ottaivacrinus, Dendrocrinus and Homocrinus since in them R' is 

 rather more asymmetrical. In these X has passed down from above 

 R, and now rests with its lower half between the right and left pos- 

 terior radials, being supported partly by R' and partly by the basal.^ 

 Carabocrimis, Botryoerinus, and similar forms are, as all acknowl- 

 edge, the next stages in the shifting of the radianal ; in these X has 

 sunk still lower into the dorsal cup, and is now entirely in a line 

 with the radials." " In Parisoerinufi and Euspirocrinus among 

 pinnuless forms, and in the Poteriocrinites, another change has taken 

 p)lace ; the radianal has passed through a revolution of 90°, and the 

 lowest plate of the ventral sac (t) has sunk down between R and X." 



Before we enquire into the merits of this argument, we must 

 ascertain whether the plates of the diflTerent genera which Bather 

 marked X are structurally alike, for any mistake in this regard, 

 naturally upsets the whole theory. The question is, is his plate X 

 in loerimts and Merocrimts which rests upon that marked by him R 

 or C, and that upon R' in Hybocrimis and Anomalocrinus, homolo- 

 gous with the plate oi Dendrocrinus, Homocrinus, etc. supported by 



1 The latter is not the case in Hybocrinus, in which there is no anal jilate 

 supported by the basal. 



