90 
SMITHSONIAN INSTITUTION 
The median arm is borne by the 1 . ant. R, and it varies greatly in size; it is sometimes 
much the largest, furnishing the main controlling element in the movement of the crown ; 
frequently it remains simple, extending distally beyond the ends of the other arms ; it may 
branch, usually only once in the earlier forms, but sometimes twice or even more in some of 
the later ; also it varies in the number and length of brachials, from 2 or 3 of extreme length 
to 20 or more short ones ; occasionally this arm is relatively slender, and even shorter than 
the lateral arms. But always it is a unit by itself, without any corresponding element. 
The two side arms borne by the large, simple radials, each at first symmetrically divided 
into two equal rami, are later extraordinarily developed into numerous branches springing 
from successive diminishing axillaries having unequal articulating faces, each giving off from 
the shorter, inner face a branch called an “ axil-arm,” and from the longer, outer face the 
next axillary. The latter constitute the main ramus of each of these arms, composed of a 
series of 3 to 8 axillaries, termed “ main-axils,” which lie side by side and curve around 
transversely toward the anal tube in form of an arch; the branch (axil-arm) springing from 
each main-axil on the shorter side toward the median arm gives off ramules in alternate 
succession, of which those on the inner side are often invisible from the exterior of the folded 
arm, while those on the outer side, especially the first one, may reach the height of the crown 
and form the chief visible part of the arm. This plan of arm-branching may be called the 
“ axil-arm system.” 
The base also undergoes notable modifications. In the first remove from the ancestral 
form, concurrent with the asymmetric location of the stem and the disappearance of the 
r. post, arm, the r. post, basal atrophies, and is only rarely, perhaps doubtfully, represented 
by a minute triangular relic ; thus there are left 4 BB, which form a semicircular base, hinged 
by muscular articulation to the radials, with the hinge on the straight side and the larger r. ant. 
and post. BB on the curve. All the basals are in contact with the stem as usual in crinoids ; 
the median two next to the hinge, 1 . post, and 1 . ant. BB, called the left basals, enter or touch 
the stem-facet by narrow points, and their outer sides make a reverse curve, giving a peculiar 
and characteristic outline. In the next, 3-armed, stage, with the stem shifted back to the 
median position, the 4 basals remain as before, the two left BB in contact with the stem by 
their points, and preserving the same general shape. But in the next, axil-arm, stage these 
basals are completely detached from the stem, and fuse into a single plate which is usually 
triangular with straight sides in the Silurian forms, and finally becomes curved in the Car- 
boniferous. Thus while in the Ordovician stage and its immediate Silurian successor the four 
basals are connected with the stem as in their ancestor when erect upon the stem, a progressive 
change occurs with the establishment of the axil-arm system, by which two of them are entirely 
withdrawn from contact with the stem or its facet. These differences are further important 
because they can be recognized from the isolated base alone, which is often the only part 
recovered in the fossils. 
These various stages form an evolutionary series, the members of which may for the 
present be designated as A, B, C and D. It begins in the Ordovician with Form A, having 
3 lateral arm-bearing rays, hereinafter for convenience called simply arms, the stem alongside 
the anal tube, the lateral axillary primibrachs equal-faced (in wbich three characters it differs 
from all the rest of the familv), also 4 basals unfused, the r. post, and r. ant. inferradials 
separated from one another and from the anal x by the corresponding superradials, and the 
two segments of 1 . ant. R broadly connected ; is modified in the Silurian with Forms B and C, 
having the lateral arms reduced to two, balanced, the stem shifted to the plane of the anal 
tube, the primibrachs unequal-faced ; the left basals in the former divided, and in the latter 
fused into a single triangular plate ; and it culminates in the Devonian and Lower Carbonifer- 
ous with Form D, in which, along with the foregoing characters of C, the fused 1 . post, and 
1. ant. basals finally form a curved plate as wide as the hinge, the r. post, and r. ant. infer- 
radials have come together underneath the anal x, the superradials which as the subanal piece 
