HETEROCRINIDAE 
97 
means of which the crown could swing back and forth like a door upon its hinge. The 
anatomical feature most frequently discussed in relation to such movements of the crown 
toward or from the stem, as has been very fully described in Dr. Ringueberg’s excellent paper 
on the Calc’eocrinidae, is the externally conspicuous hinge-like joint formed by deep grooves 
along the straight transverse edges of the consolidated basal and radial plates, which “ has 
externally the appearance of a gaping fissure, but is closed internally by the exact approxi- 
mation of the inner edges of the grooves.” The fissure is formed by the articular margins of 
the 1. ant. inferradial and adjoining radials meeting the apposed basals, bordered on either 
side by a broad and deep fossa, and divided by a straight line of contact, the approximated 
edges of which may form a fulcral ridge. Descriptions of this hitherto have dwelt chiefly 
upon the articular surface as usually seen from the anterior side, which occupies an equal 
space upon the faces of the respective plates. These spaces are often filled with numerous 
small, irregular plates, which have been described by Ulrich and by Rather, the latter under 
the term “ supplementary plates,” with several good figures, and which are here shown by 
figure 2oa, on plate 28. They are of a somewhat scaly appearance, not meeting by suture, and 
have a darker color and denser composition than the other calyx plates. In some specimens 
similar plates are also observed in the gaping articulations of the axillaries or brachials. Their 
office is not clear. That the hinge represents a muscular articulation, with the usual com- 
bination of muscle and ligament, there can be no doubt. 
This structure would provide the power for one movement of the crown. But to pro- 
duce a complete up and down motion from a recumbent to a partially erect position and 
vice versa, there must be a reciprocal mobility and muscular control between basals and radials 
at the opposite side. Abundant evidence of this is seen in the fossils, sometimes in well- 
preserved crowns where the base stands at a considerable angle from the radials, but especially 
in the numerous weathered specimens of consolidated bases and united radials, separated 
from one another along the line of the basi-radial contact. Instead of a sutural union between 
the basals and the two large inferradials at the posterior side, the edges of these plates are 
rounded or beveled, and there are marks of muscular attachment on the inner surfaces similar 
to those along the external fissure. Thus the opposing musculature is furnished for motion 
by way of flexure or extension either way. 
The mechanism of the hinge can be better understood by reference to the figures on 
plate 30, part of them based upon specimens composed from the detached elements above 
mentioned, and some upon crowns with the parts in position as found in the fossils. This 
structural material is chiefly of the abundant species Halysiocrinus perplexus Shumard, from 
the basal Lower Carboniferous. Figures 4 and 5 show the anterior aspect of the calvx when 
the articulated parts are opened as far as they will go, almost to the same plane, with the 
transverse grooves forming the two sides of the gaping fissure which lodged the exterior 
muscles actuating the movement in that direction. The floor of the grooves next to the divid- 
ing line is marked by bands of striae vertical to the margin. In figure 6 the exact opposite is 
seen, showing the disposition of the parts at the interior, and their appearance when fully 
extended. The joint-faces of the median and two lateral arms are shown, with their muscular 
markings, and by contrast the rounded edges of the two large radials which would be in appo- 
sition to the corresponding edges of the basals when closed, but without any sutural union. 
Here the relation of all the plates composing the calyx may be seen, as well as the cavity which 
lodges the viscera. From the margins of the apposed basal and radial plates wide bands of 
vertical striae extend inward, marking the attachment of powerful muscles by the contraction 
of which the crown was closed down upon the base and brought into the position in which 
it is usually found, as in figures 17, 18. 19. (Best seen with a low magnifier.) 
Starting with these two examples, we can readily trace the relation of the parts in several 
positions incident to different degrees of opening or closing. In figure i6a, from a complete 
crown, in the most frequent position of rest, the basals are firmly closed against the radials 
