500 FRANK SPRINGER 



tending toward the right. On the other hand, there is in the same 

 formation, and at apparently the same horizon, a form of Taxocrinus 

 in which, while the anal series is rounded and prominent, and merges 

 plainly into the perisome above, the plates of the bordering integu- 

 ment, though still pliant, are strong and heavy, and apparently 

 united to the adjacent brachials by loose suture. In these the modi- 

 fication begun in the last case has evidently been carried to a phase 

 in which the strong structures composing the Forhesiocrinus plan 

 have broken down in the anal area, and given place to the opposite 

 one. It thus happens that we are sometimes in doubt to which of 

 the two genera a species ought to be referred; and much of the con- 

 fusion and shifting of opinion as to the relations of these genera is 

 traceable to the failure to take proper account of these transition 

 forms. 



There was thus a sort of struggle for existence between the two 

 plans, and it would seem that the group adhered in the end to that 

 structure which was in best accord with its flexible characteristics; 

 and we may perhaps infer that the Sagenocrinus plan, which tended 

 more in the direction of the Camerate structure, was finally extin- 

 guished by reversion to the other one. This was fully accomplished 

 in the Kaskaskia, where there is a species described by Hall as For- 

 hesiocrinus whitfieldi,'^ and better illustrated by Miller and Gurley 

 under the name Taxocrinus wetherbyi;^ and of which Wetherby's 

 Forhesiocrinus parvus^ is a young individual — which has the habitus 

 of the highest-developed Forhesiocrinus in everything except the anal 

 side, where it is a perfect example of structure No. i. In fact, the 

 general character of the Taxocrini in the St. Louis and Kaskaskia 

 Limestones is that of a strong interbrachial structure in the regular 

 areas, combined with a weak and flexible one in the anal area, con- 

 taining the conspicuous tube and its bordering integument. 



In the first family — Ichthyocrinidae — structure No. 2, in its earlier 

 stages, was the prevailing type, viz., one large plate, with perhaps 

 the addition of a few smaller ones above it, completely filling the area. 

 Parichthyocrinus alone of the genera hitherto assigned to the Ichthyo- 



1 Geological Survey of Iowa, Part II, p. 632. 



2 Bulletin No. 6, Illinois State Museum, Plate 4, Fig. 3. 



3 Journal of the Cincinnati Society of Natural History, Vol. II, Plate II, Figs. ^a-b. 



