252 SMITHSONIAN INSTITUTION 



The mode of union of calyx plates in this species is well shown hy the figures from some 

 dissected parts of specimens. At the brachio-interbrachial and other non-articulating 

 sutures the fossae, though large, are interrupted by numerous small contact areas (PI. XXV, 

 fi§' s - 3 C > 5 C )> so that the ligamentous masses were not so large as in F. nobilis. The articulat- 

 ing sutures have fairly well-defined paired muscle-fossae, except upon the radial which 

 seems unpaired here as in other species (PI. XXV, figs. 6a, $e). 



In no species is the so-called " patelloid " structure at the brachial sutures carried to a 

 greater extreme than in this and its Burlington associate, and it was here that attention was 

 first directed to this character by Hall, as heretofore explained. In some specimens, especially 

 of the latter form, it affects the shape of the entire plate, the sinuosity beginning at the 

 margins and extending in a deep, broad curve to the middle, and the transverse elevation of 

 the plates being curved to correspond with it (PL XXVI, figs. 4, 5). 



The taper at the proximal end of the column is not so pronounced in this species as in 

 some others ; it all occurs within a short distance, and is sometimes not very marked. This 

 was the case with Meek and Worthen's variety giganteus, which they describe as showing a 

 perfectly straight column, whereas their figure shows that it diminishes from 13 mm. to 10 

 in a short distance. The orientation of the axial canal is somewhat irregular, being clearly 

 interradial in some specimens, while in others it is rather indefinite. 



The most peculiar thing about this species, in which it differs from all others of the 

 genus, is that it has only two primibrachs. This is no sporadic variation. It is constant in 

 every ray of every well-preserved and normal specimen known to me. Hall, in the explana- 

 tion of his figures of the types on plate 7 of Bulletin 1 of the New York State Museum, 

 distributed by him in 1872, speaks of some irregularity in these plates, as varying from two 

 to four — meaning all the primary plates including the radial and the first axillary. Those 

 with the latter number I have separated specifically for reasons which will follow. Those 

 with the former may be abnormal in one or more rays, although I have not seen any such 

 among my specimens, while I have some crushed in such a way as to give the impression of 

 only one primary plate in a ray above the radial, which was always corrected by closer 

 inspection or further cleaning. I have before me 14 specimens in which 55 rays are visible 

 to the first axillary, and there is among them not a single exception to the number of two 

 primibrachs. Meek and Worthen's variety giganteus is clearly abnormal in the ray from 

 which the figure was drawn, and there are fractures about the base which were not correctly 

 interpreted by the artist. Neither is there any exception among the specimens which I have 

 separated on account of having three primibrachs, save in one young individual. 



It would seem, therefore, that this genus, evolving from a Silurian form like Sageno- 

 crinus or Temnocrinus, underwent its final transformation in the Burlington limestone by 

 taking on an additional primary plate in each ray ; and that this was accomplished by a 

 specific change far more complete and decisive than we find in some other cases where the 

 modification was going on in the Devonian. Doubtless if we had all the stages of this 

 development through the Devonian we should see phases of individual variation ; but the 

 final stage became firmly established in the genus, with one species tenaciously holding on to 

 its Silurian type of ray structure. It is singular that with this apparently straight line of 

 derivation, no trace of this type has been found in the prolific crinoidal deposits of the 

 Lower Burlington beds at the same locality. We must remember, however, in this and all 

 similar cases what we are constantly prone to overlook — that our collections, even from 

 these prolific and well searched formations, represent only a mere atom of what actually 

 existed during their epoch, or was deposited in their sediments. 



Types. Hall's original is in the University of Chicago ; the other specimens figured are 

 in my own collection. The original of figure 2, Plate XXV, one of the finest specimens 

 known, was found by Mr. J. O. Beebe, a most zealous and intelligent Burlington collector, 

 who presented it to me for use in this work. 



Horizon and locality. Lower Carboniferous, Upper Burlington limestone; Burlington, 

 Iowa ; Henderson County, Illinois ; and Washington County, Indiana. 



