THE CRINOIDEA 



and that the proximal median plate of locrinus and Heterocrinidae 

 (which they call t) is represented in genera with a special anal x 

 by a tube-plate (rt) on the left shoulder of r. post. R (Fig. XXVI. 

 2-6) ; they assert that "in the earlier and simpler forms, the tube 

 consists of only five [vertical] series, one to each interradius, that 

 of the anal side resting upon t. Later on, as the tube grew larger, 

 a new row of plates was introduced with plate x supporting it. 

 When there are three series [at the posterior side], as in Dendro- 

 crinus, the third generally rests upon one side of the left posterior 

 radial. The arrangement of the plates within the rows is so 

 regular, that if a sinking of the plate t had taken place, it would 

 certainly be indicated by some disturbance among the lower plates 

 in the tube." In this sentence the proximal median plate t (our x) 



FIG. XXVII. 



Anal area of Hetero- 

 crinus isodactylus, 

 showing close connec- 

 tion of anal (2) with 

 right posterior super- 

 radial (r.p.Ii). En- 

 larged. (From Bather.) 



FIG. XXVIII. 



locrinus, showing connection of anals and brachials. 1, 

 part of the r. posterior ray seen from inside the cup ; 2, 

 upper articular surface of Ri ; the groove on the left goes to 

 .), that on the right to brachials ; 3, left upper articular 

 surface of Its, supporting x ; 4, right upper articular sur- 

 face of lis, supporting IBi'i. Enlarged. (From Bather.) 



of Heterocrmus is identified with rt of Dendrocrinus ; but no proof 

 has yet been given that the added series may not be those starting 

 from rt and It, rather than from x and It. On the other hand, the 

 view that the series x is homologous with the series t, is supported 

 by the general size and appearance of the two, and by the inferred 

 relations of the axial cords. And the homology of x with t is 

 supported by the facts that the position of t with reference to r. 

 post. Rs, does vary from a higher to a lower level in early genera, 

 while the position of x to the adjacent radials likewise varies. 

 In late Carboniferous genera of Dicyclic Inadunata, x certainly 

 appears to pass up out of the cup (Fig. XXIX.), and this inter- 

 pretation is confirmed by the migration of the anal in the develop- 

 ment of Antedon, which anal is universally homologised with x 

 (Fig. XXX.) ; but if a plate can pass up, it can also pass down, as 



