“PLATYCRINIDZE. © 681 
longing to one species. In all of them there appear around the stem three 
conspicuous nodes, which are continued to the upper edges of the basal disk ; 
and in all may be seen the unique articulation between the lower brachials 
and arm plates; while the inflation of the arms at their middle portions is a 
frequent character in mature specimens. But far more striking are the dif- 
ferences caused by growth. Comparing the smaller specimens with the 
larger ones, it will be found that the arms in the smallest or youngest ones 
are not only uniserial throughout, but decidedly zigzag, and that their joints 
are proportionally much longer than those of the adult. In more advanced 
specimens the tips of the arms are biserial, and the biserial part extends 
downward, as the specimens grow larger, by the introduction of additional 
joints at the top, and the progressive interlocking of the cuneate plates. 
The modifications here exhibited, and due beyond all question to individual 
growth alone, are so remarkable, that without a knowledge of the ontogeny 
of the Crinoids, the earlier stages of this species might readily be taken as 
representatives not only of different genera, but even of different groups. 
Platycrinus boonvillensis 8. A. Minirr. 
Plate LX XII, Figs. &a, b, and 9. 
1891. S. A. Minter; Bull. 4, Geol. Surv. Missouri, p. 8, Plate 1, Figs. I and 2. 
A rather large species of the type of P. burlingtonensis. Calyx to the 
top of the radials bowl-shaped, wider than high, slightly pentagonal as seen 
from above. Plates moderately heavy, the surface smooth, or nearly so; 
the basi-radial and interradial sutures grooved. Basals closely anchylosed, 
the lines of union elevated into ridges; they form a low, rapidly spreading 
basin, distinctly pentangular at the upper end, broadly truncated at the 
lower, the bottom deeply excavated so as to form a rounded, rugose rim 
around the column. Column facet circular, occupying one half of the width 
of the concavity, its face covered with radiating striz. Radials a little wider 
than long, gradually expanding upwards, slightly more elevated along the 
median line, and somewhat beveled toward the sutures. The facets occupy 
less than half the width of the plates; they are shallow, directed upwards, 
and surrounded by a projecting rim. - 
Horizon and Locality. — Lower part of Warsaw limestone; Boonville, Mo., 
and Spurgen Hill, Ind. 
Types in the collection of Mr. Sampson. 
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