PETALOCRINIDAE 
127 
about 20 mm. Arm-fans bilaterally symmetrical in number and distribution of 
grooves; average of 30 isolated fans gives length 10.6, width 13 mm., with 
range of variation from 7.75 to 14.5 mm. length and 7 to 16 mm. width. Ventral 
grooves, finial, mostly 16, rarely 14. Angle of arm-fans 51° to 83°, normal 
variation 71° to 78° ; ridges usually narrower than the grooves. 
The above figures are taken from Bather’s tabulation of a large number of specimens. 
They are confirmed by a series of about 80 fans in my own hands which indicate a range of 
18 to 30 mm. in diameter of crown, with slight difference of average in some other respects. 
Combining the data there is thus established a limit for the species not exceeding 14 mm. 
length and 16 mm. width of fan, and not less than 50° nor more than 83° angle of the sides. 
A fair representation of its form and proportions is given by the figures on plate 26. Figure 13 
shows the smooth, amorphous dorsal surface of a set of isolated fans, artificially arranged, 
and figure 12 the sharply grooved r^entral furrows of another set. The ten pieces have a 
range of 10-13 mm. length and 10-14 mm. width. It is therefore a relatively small species. 
In figure 14 we have a double size view of the complete calyx with all the arm-fans in posi- 
tion, but the dorsal side is stripped off, leaving only the inner surface of the ventral grooves 
visible. 
There is a very close resemblance between this species and the Gotland form, P. visby- 
censis of Bather, which is of similar small size and number of grooves, but has a greater range 
of variation. 
Horizon and locality. Hopkinton stage of Niagaran ; near Monticello, Jones County, 
Iowa. The original of figure 14 was collected by the late Professor Samuel Calvin, in 1899. 
Petalocrinus inferior Bather 
Plate 26, figs. 15-16 
Petalocrinus inferior Bather, Quart. Jour. Geol. Soc. London, 54, 1898, pi. 26, fig. 57, text-fig. 10. 
From the American material loaned him Bather described two new species, 
P. inferior from the type locality in Jones County, Iowa, and P. long us ''from 
St. Paul, Indiana, each based upon a single arm-fan in the Walker Museum, 
University of Chicago, from approximately equivalent Niagaran horizons, the 
former perhaps a little the higher. 
Among my collections from the Laurel formation at St. Paul are four 
specimens of arm-fans of dimensions far exceeding those of the type species, 
but which at once invite comparison with the two above m.entioned. Three of 
them exhibit remarkable uniformity of characters, as shown by the following 
data : Length of arm-fan 23, 28, 28 mm. ; greatest width 22, 25, 26 mm. ; num- 
ber of finial grooves 30, 30, 28; angle of fan 70°, 65°, 68°. The grooves are 
divided into two closely symmetrical halves. 
Two of these specimens are figured; the first, plate 26, figures 15, 15a, enlarged, is a 
very well preserved fan with the grooved ventral and smooth dorsal surfaces exposed free 
of the matrix. The second, figure 16, natural size, shows only the ventral surface; and in 
both the division of the ventral grooves into two symmetrical halves, and the paddle-shaped 
handle of the fan at the proximal end, are well marked, as they are also in the third specimen 
of about the same size and form, not figured. 
