HETEROCRINIDAE 
99 
expressed by statements such as “ the crinoid hangs downward along the column,” and “ the 
upper part of the crinoid could be bent upward and brought into an erect position.” Such a 
lifting of the entire crown upon so small a fulcrum would seem to call for a muscular equip- 
ment rather greater than is indicated by the evidence in the fossil state. 
A more recent interpretation has been given by Jaekel (Phylogenie und System, 1918, 
pp. 86-88) , which seems to accord better with the observed facts. He considers these crinoids, 
which he groups as a suborder Calceocrinites, to be reef-dwellers, living exposed to tidal 
currents, the stem lying recumbent upon the bottom, where when at rest the crown was closed 
down at one side like a barnacle. When feeding, the arms opened out toward the current, for 
which but little movement of the crown as a whole upon the base was required. When the 
tide turned, the arms would readily close down, the larger ones serving as a protection for 
the more delicate parts against injury from the ebb-current. Thus the crown habitually was 
in a horizontal position to start with, from which only a relatively small movement was 
needed beyond the ordinary opening of the arms to enable the organism to obtain food from 
above like other crinoids. The theory is graphically presented by Jaekel’s figures, 81, 83, 
representing the closed and open conditions. 
This interpretation presupposes a sessile condition by means of a distal attachment of 
the stem to the reefs. Some evidence of this is seen in specimens from Dudley, England, in 
which the stem is sometimes found rather short with a foreign object attached. In the Ten- 
nessee specimens it is not so evident, the few stems preserved being fairly long, and apparently 
tapering towards a point. 
With the recumbent position as that of rest, in which the crinoid habitually reclined 
except when feeding, the diminution in size of the arms toward the posterior side, which 
became such a marked character of the post-Ordovician forms, is accounted for as the result 
of a cramped position, pressure, and loss of motion. 
Evolution in the Act 
Returning to the evolutionary modifications exhibited in this group, the study of the 
material now in hand Iras brought to light some remarkable facts. The most striking move- 
ment within the family, next to the development of the axil-arm, was the elimination of the 
fourth arm from Form A, concurrent with the shifting of the stem back to its normal posi- 
tion in the plane of the anal tube, and tbe restoration of bilateral symmetry in the resulting 
3-armed crinoid, thus establishing a condition in which all the other forms differed in a large 
way from the typical Ordovician form. Such a change in characters of major importance, 
involving some of the larger factors in the anatomy of the group, would be expected to occur 
in a gradual way, scarcely discernible in individual cases. Hence we were prepared to observe 
results rather than the process by which they were obtained. 
When preparing the finely preserved Tennessee material belonging tO' this form, I was 
surprised to find in some instances a marked difference in the size of the lateral arms. Upon 
following this up in seven of the nine specimens herein described under three species, it was 
disclosed that in every one of them the right anterior arm was smaller than the anterior arm 
adjoining it. The diminution was in various degrees, in some cases amounting to well-defined 
atrophy by which the arm was reduced to' less than one fourth the size of its adjacent fellow. 
The reduction is decisively shown by the figures on plate 28, especially figs. 3^; and in 
which the smaller arm occupies but a small part of the space on that side ; in the former 
specimen its branches are like very short pinnules, in contrast to the long and robust ramules 
of the other arm alongside ; and in the latter it lies against the large anal tube with scarcely 
more than a remnant of its primitive dimensions. In another species, figure 7 of plate 28, 
having both arms spread like a fan in excellent position for observation, the area of the 
anterior arm is about five times that of the dwarf. In every case, with the two lateral arms 
of the anterior side lying side by side for comparison, the difference in general dimensions. 
