49a HERRICK E. WILSON 



It is evident from those observations that the Dumber of basals was gradu- 

 ally reduced in Paleozoic times, and that in the Camerata the anal plate was 

 introduced after the quadripartite base had made its appearance. It will 

 now be shown that this diminution of number was the result of fusion of two 

 or more of the live original plates, and that by the introduction of the anal 

 plate the base underwent further modifications. The manner in which the 

 modifications in the number of basals were effected may be best understood 

 by reference to the diagrams on Table A. [This table is photographically 

 reproduced as Fig. i of this article.] 



looking at these diagrams, the transmutation in the Camerata from five 

 basals to a less number is readily understood among genera in which the anal 

 plate is wanting. When the base is quadripartite, it is invariably the two 

 anterior plates of the elementary five which are consolidated (a). In the 

 tripartite base there is a fusion of the posterior with the left posterodateral 

 basal, and another between the right posterior and the adjoining antero- 

 lateral plate (3). The figure shows that a bisection of the two larger plates 

 will reproduce the original five pieces, interradially disposed. 



The case is not so simple in genera with an anal plate, where the form of 

 the basal disk is changed from pentagonal to hexagonal (4), as a bisection of 



smallest anal plate, now known among Camerata in which the anal plate is in 

 apposition with the base. See PI. 111. No. n. and Ret. 39, PL 70. Figs. j-q. 



When preparing figures for this paper from specimens in Mr. Springer's colh 

 with his permission. 1 overlooked the fact that the dorsal cup of this species had never 

 been described or figured. As to this. Mr. Springer has furnished me the following 

 note: 



"When proposing the species Pterotocrinus " Kentucky. Ill . 



4-0. PI, 1. Figs 1,:, b) I. yon described and figured only the tegmen with the ponderous 

 wing plates, as did Wachsmuth and Springer after him (NJB. < 

 While this work was in press I discovered in the Museum of Comparative Zoology 

 a: Harvard a lead cast of what was apparently the same specimen, with the dorsal 

 cup attached, which fact 1 mentioned in a footnote on the page cited. When I acquired 

 in 1903 the collection of the late Col. Sidney S. l.yon. I found associated with the 

 tegmen constituting the published type the dorsal cup reproduced in the east; the 

 two parts were separated, but I have again united them. It is probable that they 

 belong to the same specimen, and the fact that they pertain to the same species is 

 proved beyond question by another specimen from the same locality having the same 

 dorsal cup with one of the wing plates attached. The species is remarkable, not only 

 for its extraordinary wing plates, but also for the construction of the dorsal cup. in 

 which the basal plates are very small and tlat. while the radials are of enormous 

 larger than all the other plates of the cup combined; this being the reverse of the 

 structures in Pi and all other known species of the genus. It occurs in the 



Birdsville formation of the Kaskaskia group, in Crittenden County, Kentucky, where 

 it is extremely rare: and also in the Renault formation in Monroe County, Illinois, 

 from which region a fragmentary specimen was described by Hall [Geo/. Towa, 11 v 

 PI. XXV. Fig. 7^ under the name Dit 



