92 
SMITHSONIAN INSTITUTION 
before separated them having disappeared, and the two segments of 1. ant. radial, having 
as a rule become disconnected in C, are now widely and permanently separated by the two 
large radials meeting between them. In B the lateral arm structure is of an intermediate 
stage, while C, which is abundantly represented in the fauna herein described, has in common 
with D the axil-arm structure, and differs from it in the fact that in most species the two 
inferradials are still separated by the subanal piece formed by the fusion of their respective 
superradial segments, and also in the size and shape of the fused basals. 
This may be better understood by reference to the accompanying series of diagrams on 
page 91 constructed as if the cup were opened between the anterior and left anterior radials 
and spread out flat ; this puts the left anterior ray always at the left end, with the anal tube and 
stem at the front. The relative position of the respective parts is maintained throughout. 
In the Ordovician type, which passes over into the Silurian, the side arms branch so that 
the ray divides equally upon the axillary primibrach into two main divisions, which are sym- 
metric as between themselves. From these ramules are given off from successive brachial series 
alternately on opposite sides after the usual manner of the heterotomous arm. 
In the other leading Silurian form, C. as before stated, the axil-arm system, a wholly 
different and unprecedented plan of arm structure, was developed, in which, instead of the rays 
dividing symmetrically into two equal rami, they divided upon the primibrach from unequal 
articulating faces, the shorter one giving off an axil-arm and the longer one another axillary, 
likewise unsymmetrical, from which the same process was repeated several times. Thus the 
branching was by successive axillaries from one side of the ra}^ outward from the median arm 
toward the anal tube, producing a somewhat fan-shaped arrangement of the ray on either side. 
This modification changed the general aspect of the crinoid, and the relation of its calyx and 
arm plates, to such an extent that in order to understand the descriptions it is desirable to have 
a little clearer account of what happened during the course of the evolution from Ordovician 
to Carboniferous times. 
Starting, for example, with the cup of Heterocrinus hellevillensis of W. R. Billings, as 
given opposite O at the bottom of the series, we have a circlet of five arm-bearing radial plates, 
three of them compound and two simple, together with an anal tube springing from the 
r. post, radial, all underlain by 5 basals. The plane of bilateral symmetry in this circlet bisects 
the anal tube and the simple anterior radial, with two radials on either side, one simple and 
one compound at the left, and two compound ones at the right. By reason of the sternward 
bending of the crown, and consequent pushing of the anal tube to the right, the two arms to 
the right of it were one after the other crowded out ; the superradials and inferradials were 
modified until the arm-bearing segments disappeared or ceased to function, and there were 
left only three arm-bearing radials, the 1. post., 1. ant. and ant. RR in that order. The 1. ant. R 
became the base of the median arm ; the other two, having greatly increased in size, supported 
the side arms, which by their numerous branches spread around toward each other, and 
toward the anal tube, usually overlapping and concealing it from view. This resulted in a 
flattened or elliptic ring of calyx plates, consisting of a median and two lateral arm-bearing 
radials and the anal series, in which the radial antipodal to the anal tube was now the left 
anterior instead of the anterior as in the ancestral stage, leaving one of the large simple radials 
at either side ; so that the plane of bilateral symmetry would lie from the stem and anal tube 
to the median arm. 
Thus the goal toward which this family group was tending was a tri-radial form with a 
perfect bilateral symmetry. It was attained by (i) complete elimination of the ancestral 
r. post, and r. ant. rays through atrophy of their superradials and incorporation of their infer- 
radials with the anal structures; (2) restoration of the stem to the plane of the anal tube; 
along with these occurred (3) the equal development of the axil-arm system on either side 
of the median arm ; and (4) the permanent detachment of 1. ant. inferradial from the ray by 
the meeting of the two large radials above it. 
