66 SMITHSONIAN INSTITUTION 



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

 strong structures composing the Forbesiocrinus 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 confusion and shifting of opinion as to the relations of these 

 genera is traceable to the failure to take proper account of these transitional 

 forms. 



There was 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 struc- 

 ture, was finally extinguished by reversion to the other. This was almost 

 accomplished in the Kaskaskia, where there is a species, Taxocrinus zvhit- 

 fieldi Hall, which has the habitus of Forbesiocrinus in everything except the 

 anal side, where it is a perfect example of structure No. i (PL LX). In fact, 

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

 stones is that of a strong interbrachial structure in the regular areas, com- 

 bined with a weak and flexible one in the anal area containing the conspicuous 

 tube and its bordering integument. In the latest surviving genus of the 

 Sagenocrinidae, Amphicrinus, which continues into the Coal Measures, there 

 is a similar tendency. Not only is the anal interradius persistently narrower 

 than the others, but it shows signs of breaking down through some instability 

 of the lower plates which appears in nearly all the specimens (PI. XL, 

 figs, ga, b). 



In the family Lecanocrinidae structure No. 2 in its earlier stages was the 

 prevailing type, viz., one large anal plate, with perhaps the addition of a few 

 similar ones above it, completely filling the area. In Lecanocrinus macropet- 

 alus, from the Niagaran group at Lockport, New York, we have a most charac- 

 teristic example of this family — a rounded, ovoid crown, with a perfectly even 

 surface, the rays and arms flat dorsally and in close contact throughout 

 (PI. Ill, figs, i to 19). It has a single very large plate rising above the level 

 of the radials, and curving to a point between the rays, which abut upon it at 

 either side and close over it in an arch. This plate is without depressions of 

 any kind, and is entirely flush with the general curvature of the crown. In 

 certain other species from the same locality and horizon, now referred to the 

 new genus Asaphocrinus (PI. IV), this plate begins to be depressed at the sides 

 from the upper end down, so as to leave a ridgelike elevation in the middle 

 quite resembling the base of our so-called anal tube, while the full dimensions 

 of the plate are still retained. In some specimens of these forms we can see 

 this median ridge continued by a second small rounded plate of the same form 

 and size, the large anal having lost its pointed angle and become truncate to 



