680 HERRICK E. WILSON 



Devonian expression of Hexacrinus, as the reversion from a cam- 

 erate type of tegmen to the flexible type, and a redevelopment of 

 the camerate type as shown in later forms of Dichocrinus and more 

 strongly in its descendants, Talarocrinus, etc., are scarcely probable. 

 When, however, Dichocrinus (PI. Ill, Nos. 12, 15) is compared 

 with a Kinderhook species of Platycrinus, namely P. symmetricus, 

 (PI. Ill, Nos. 13, 14), such a remarkably close parallel in calyx 

 structure is noted that with the insertion of an anal plate and 

 proper modification of the base P. symmetricus could scarcely be 

 distinguished in calyx structure from Dichocrinus inornatus. How- 

 ever, some species of Dichocrinus have uniserial arms, and all have 

 a circular stem. Dichocrinus could not then have originated from 

 the immediate ancestor of P. symmetricus, but from a somewhat 

 earlier stage where the stem was circular, the arms uniserial, and 

 the base of the ab — c — de — ■ type. Hence a new theory for the 

 origin of the bipartite, hexagonal base is suggested: 



Dichocrinus and its descendants probably originated from some 

 genus of the Platycrinidae which had a circular stem, flexible 

 tegmen, branching uniserial pinnulate arms, and a base of the 

 ab — c — de — type. The hexagonal, tripartite base originated by 

 interpolation of the anal plate in the radial cycle by portional 

 migration, closure of the left-anterior suture, partial atrophy of 

 the right side of the posterior basal, and a compensatory hyper- 

 trophy of the left side of the right-posterior basal which shifted 

 the right-posterior basal suture to a posterior position. This 

 metamorphosis is expressed in changing formula ab — c — de — to 

 abc — dex — (Fig. 9, Nos. 4, 4a, b). 



4. THE SUCCESSION OF BASAL CHANGES IN THE PLATYCRINTDAE 

 AND THE HEXACRINIDAE 



The succession of basal changes in the Platycrinidae and the 

 Hexacrinidae seems, from the evidence herein offered, to have 

 followed a different line from that postulated by Wachsmuth and 

 Springer, and the following succession is suggested. 



The earliest Platycrinidae, as shown by the ontogenetic develop- 

 ment of Platycrinus, had a simple pentagonal base of the a — b — c — 

 d — e — type and a circular stem. Before the Niagaran (Silurian) 

 this genus gave rise to the Platycrinus type of base {ab — c — de — ) 



