194 ‘THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
last surviving genus of the Hexacrinide; and it is most interesting to find 
here again emphasized the truth which Palzontology teaches us by so many 
examples, that extravagance of form and rank development in any group is 
the signal for its speedy extinction. 
Pterotocrinus capitalis (Lyon). 
Plate LX XIX. Figs. 6a, b. 
Se a 
1857.  <Asterocrinus capitalis — Livon; Geol. Rep. Kentucky, Vol. III., p. 472, Plate 3, Figs. 1, la-#. 
1859. Pterotocrinus capitalis — Lyon and Cassrpay; Amer. Journ. Sci. Vol. XXIX., p. 68. 
1881. Péterotocrinus capitalis— W.and Sv.; Revision Paleocr., Part I1., p. 91. 
Crown as viewed from above irregularly star-shaped, in profile resembling 
the form of a Corinthian capital. Dorsal cup about as wide as high, the 
plates heavy and without ornamentation. 
Basals proportionally larger than in any other known species of the 
genus, forming a rather deep basin, almost as large as the remaining portions 
of the dorsal cup together; the sides rounded, contracting at the upper end, 
and somewhat flattened at the bottom; the median portions slightly concave, 
with a deep pit in the centre, completely filled by the column. The upper 
margin of the basal cup is indented for the reception of the radials, and 
deeply notched for the anal plate. The latter extends to the full height of 
the radials, but is considerably narrower; it is trapezoidal, the upper angle 
being acute, the lower one obtuse. Radials very short, their width almost 
four times their height; the lower face convex, the upper excavated by the 
radial facets. Costals larger than usual in the genus, wider than long, trian- 
gular; the lateral faces a little concave. Distichals twice the size of the 
costals, wider than long, the upper angles obtuse. Fixed palmars 1 X 4, in 
contact laterally; the two outer ones resting with one of their lower faces 
upon the radials, with the other against the distichals; the two inner plates 
of the rays supported exclusively by the distichals; all succeeding brachials 
free. Arms four to the ray, short, rather strong, tapering to the tips, and 
incurving. They are biserial from the second plate, and the two series are 
united by a zigzag suture. Structure of the greater part of the ventral disk 
not known. The orals, of which portions are exposed, are elongate and 
almost —some of them completely — separated by the wing-like appendages. 
The latter, as viewed from the side, are elongate subtrigonal, with rounded 
outer angles; they are massive, deep, and flattened at the sides, thicker at 
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