260 SMITHSONIAN INSTITUTION 



beyond the tertibrachs, but from their great diminution in size I have no doubt that the arms 

 became small and infolded at a level but little above that shown in figure la of Plate XXXI. 



The truncate posterior basal, with one plate following instead of two, has become a con- 

 stant character in this species, being uniform in all the 25 or more specimens showing these 

 parts; only one from the same locality (PL XXX, fig. 17) has an angular basal, and it is 

 probably not of this species. There is also a peculiar modification in the orientation of the 

 small infrabasal, which is almost uniformly located at the anterior instead of the right pos- 

 terior as usual in the Flexibilia ; among the numerous specimens only one has it in the 

 regular position, while in two it is at the left posterior (PI. XXX, fig. 10) ; in one of the 

 latter this plate is larger than either of the other two. Being the last known survivor of the 

 genus, strongly divergent from the usual form of the calyx, and very abundant at the typical 

 locality, the species presents just the conditions for variation even in characters belonging 

 to the entire group. 



The species was described by Hall in the Supplement to the Geology of Iowa, without 

 illustration, from a single specimen found by Professor J. M. Safford, formerly State Geolo- 

 gist of Tennessee, in the upper part of the Keokuk exposures on White's Creek, near Nash- 

 ville, Tennessee. His description was very clear, and enabled me to recognize the species 

 readily from an unlabeled cast in the Museum of Comparative Zoology, which afterwards 

 proved to be taken from the type. No other specimens were found at the original locality, 

 and the type, packed away after Professor Hall's description of it, remained inaccessible until 

 recently. In the meantime a considerable number of fragmentary specimens were found in 

 the debris of the so-called Warsaw beds at and near Spergen Hill, associated with Pen- 

 tremites conoideus and Batocrinus icosidactyhts, which upon comparison with the type have 

 proved to belong unquestionably to this species. Only two, and these quite small, have any 

 considerable part of the calyx intact, but among the detached bases are some even larger 

 than that of the type. The specimens were in a loose matrix readily removed by washing, 

 and have furnished important information as to the structure of the inner floor of the calyx, 

 and the axial canal, which is discussed in the remarks on the genus. A representative series 

 of them are figured on Plates XXX and XXXI. 



These specimens also furnish fine examples of the mode of union of the calyx plates in 

 this genus ; that of the brachio-interbrachial sutures with their numerous small, interrupted 

 ligament fossae on Plate XXXI, figures 6c, d, yd, and that of the brachial articulations with 

 their paired muscle-fossae, figure jc; also for both, figures 10a, b. In figures 3b, c the 

 articulation on the radial is unpaired. Figures 6a, b also show the manner in which inter- 

 brachials grew from the inside of the calyx to fill up space as the visceral mass increased ; 

 comparison of the lettered plates in the two figures will show how much larger these plates 

 are at the interior than at the exterior, while the numbered plates show how the reverse is the 

 case with the brachials, which, being the first developed, are gradually wedged apart from 

 the inside by insertion of the newly forming interbrachials. 



Unfortunately none of the specimens have the arms preserved, the highest plate of the 

 calyx remaining being IIIBr . From the great width of the calyx, and the rapid contraction 

 and small size of plates above the secundibrachs, it is probable that the crown was relatively 

 very low and short armed, as in F. greenei. In the calyx alone, however, it is by far the 

 largest known crinoid of the Flexibilia, the type specimen being about three inches in diame- 

 ter ; and there are fragments indicating the existence of others at least half an inch wider. 



Types. In the author's collection, the original of Plate XXXI, figures la, b, having been 

 presented to me by Professor Safford when rediscovered a few years ago. 



Horizon and locality. Lower Carboniferous, highest part of the Keokuk limestone ; 

 White's Creek, Tennessee, and at the base of the so-called Warsaw beds at Spergen Hill and 

 vicinity, Indiana. 



