126 
SMITHSONIAN INSTITUTION 
Family PETALOCRINIDAE Weller and Davidson 
Eistulata in which IBB are minute and probably fused, and in which the 
branches of each arm, from lAx to the finials inclusive, are fused into rigid 
arm-fans, articulating with the cup by means of a free IBr. No anal. 
Petalocrinus Weller and Davidson 
Plate 26 
Petalocrinus Weller and Davidson, Jour. GeoL, 4, 1896, pp. 166-170. — Bather, Quart. Jour. Geol. Soc. London, 
54, 1898, pp. 401-441. pis. 25, 26; Treatise on Zool., pt. 3, 1900, p. 175, fig. xci.— Thomas, Proc. Iowa 
Acad. Sci., 22, 1916, p. 289. 
A dicyclic Inadunate crinoid, with rays multibrachiate, fused into rigid, 
fan-shaped appendages, smooth and amorphous at the dorsal side, and grooved 
for the ambulacra at the ventral, articulated to the radials through a single 
short primibrach; with pentamerous symmetry; IBB minute, hidden by column; 
5 BB similar in form; RR 5, equal, with no anals. Stem subcircular. 
Genotype. Petalocrinus mirabilis Weller and Davidson. 
Distribution. Silurian ; America and Gotland. 
In the fusion of its many-branched arms into solid appendages this genus differs from all 
other crinoids. Described by the discoverers, Weller and Davidson, in 1896 from the Silurian 
of Iowa, it later formed the subject of an elaborate memoir by Bather in 1898, based upon 
the type and other material from America loaned to him for study, together with specimens 
from the corresponding horizon in Gotland. From the latter he described three new species 
and from the former two, making a total, including the type, of six species. To his complete 
and exhaustive discussion of the morphology and systematic relations of this unique form 
I would refer the student for all needful information. 
The genus is introduced here because my collections from St. Paul, Indiana, have yielded 
several excellent specimens confirmatory of one of Bather’s species. For the better under- 
standing of this, I am giving some instructive illustrations of the type species, P. mirabilis, 
especially one of an unusually fine specimen from the original locality in the rare condition 
of having the calyx and appendages, which are called arm-fans, in place. For the use of this 
I am indebted to the thoughtfulness of Prof. A. O. Thomas, of the University of Iowa. 
Of the six described species, all but the type are represented only by the isolated arm- 
fans ; and of the type, while these fragments are numerous, only three or four specimens 
have been found with the fans in position and the calyx preserved. From this lack of other 
elements for comparison, specific characters must be sought in the relative proportions of the 
arm-fans, their shape as determined by the angle formed by their sides with one another, and 
the number and distribution of the ventral grooves. The fans are somewhat paddle-shaped, 
and the facet for articulation at the narrow proximal end is almost the width of the radial. 
Petalocrinus mirabilis Weller and Davidson 
Plate 26, figs. 12-14 
Petalocrinus mirabilis Weller and Davidson, Jour. Geol., 4, 1896, p. 166. — Bather, Quart. Jour. Geol. Soc. 
London, 54, 1898, p. 427, pi. 26, figs. 37-56. 
Dorsal enp bowl-shaped, flattened dorsally, height about half its width 
which varies from 3.25 to 6 mm. ; diameter of crown with fans outstretched 
