1890.] NATURAL SCIENCES OF PHILADELPIIIA. 385 



troduced between the radial and the plate X, supported by the radi- 

 anal — the former R'. 



The derivation of the Poteriocrinidae, etc. was explained by Bather 

 as follows : " in the Poteriocrinites another change has taken place; 

 the radianal has passed through a revolution of 90°, and the loAvest 

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

 are somewhat in doubt what Mr. Bather means by " revolution of 

 90°," but suppose it refers to the change in the position of the radi- 

 anal toward the right posterior radial. If he means that here the 

 radial moved away from the radianal, we agree with him ; but if it 

 implies the radianal shifted away from beneath the radial, we can- 

 not follow him, for that plate, as already stated, retains its position 

 throughout the different phases of its development, and only under- 

 goes modifications in size and form, until it disappears entirely. 

 That the lower part of the tube "sunk down between R and X" ap- 

 pears to us doubtful. It is more probable that a new plate was in- 

 troduced beneath the other, a sort of third anal, and this is partly 

 confirmed by the fact that the plate is imperforate. 



This explains the anal structure of all Poteriocrinidae except that 

 of Ulocrimis, Graphiocrinus, Ceriocrlnus and Erisocriiius, which we 

 regard as transition forms toward Eacrinus. The ventral tube, 

 which through these forms dwindles down again to a short cone, no 

 longer necessitated the same representation in the dorsal cup, and as 

 the anal plates gradually diappeared, the posterior radials assumed a 

 symmetrical position. In the new genus Ulocrinus of S. A. Miller 

 the plate X ceased to be represented, and its place was taken by the 

 radianal ; while in Graphiocrinus and Ceriocrinus only the plate X 

 is represented. In Graphiocrinus, in which the ventral tube is quite 

 capacious, the plate X is comparatively large ; while in Ceriocrinus, 

 with a small tube, it is reduced to a very narrow piece, which rests 

 upon the greatly extended posterior basal. In Erisocriiius, Stemma- 

 tocrinns and Encrinus the latter plate also disappeared, and the 

 tube came to rest upon the edges of the radials, where it started in 

 Hyhocriims, Ectenocrinus and allied forms. 



From the above considerations it aj)pears to us that Bather's the- 

 ory respecting the " brachianal" is based upon a misinterpretation 

 of the plates. If it were true that the plate resting to the left of the 

 radial of locrinus, passed in later forms down to thebasals, it would 

 mean nothing less than a partial revolution of the entire tube to the 

 left. Consulting the specimens it will be found that in this genus 

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