IIETEROCRINIDAE 
93 
Such is the general type of the prevailing post-Ordovician forms, of which there is 
herein illustrated a series of typical Silurian and Carboniferous species. Orienting the crinoid 
in the usual way by viewing it from the anal side, the large anterior radial would be said to 
lie at the right side, and the left posterior at the left ; each supports a many-branched arm, 
and between them is the left anterior, median radial supporting its arm of different type which 
is frequently unbranched. But for descriptive purposes, on account of the very peculiar mode 
of branching of these side arms in Silurian and later forms, it has been found more con- 
venient to view the crinoid from the conspicuous median arm, and while retaining the desig- 
nation of the arms themselves as 1. post, and ant., according to the conventional plan, to reckon 
direction of their component parts from the median arm outward and around toward the anal 
side. Therefore from this view the anterior arm lies toward the left, and its branches as seen 
from the exterior diminish in size from the median arm sternward, such branches as are 
given off in that direction being termed in the literature “ adanal,” “ outer,” or “ left,” while 
those that lie toward the median arm, and therefore away from the stem and anal side, are 
termed “ abanal,” “ inner,” or “ right.” In describing the corresponding details of the arm 
at the opposite side (left posterior) these “ right ” and “ left ” directions are reversed. 
The Axil-arm System 
With this distinction borne in mind, some of the confusion in the descriptive literature is 
clarified, and we are now in a position to give a more detailed account of the mode of branch- 
ing which was finally attained in the family, and which forms one of its most striking charac- 
ters. In this later and final stage of arm structure the axil-arms, borne upon the outer and 
shorter faces of the main-axils, are composed of a succession of brachials in series of two 
or more each, called for convenience Alphabrachs, Betabrachs, Gammabrachs, etc. The distal, 
or upper, Alphabrach is an axillary, of which the inner articulating face bears a ramule, rarely 
seen ; the outer face carries the main arm with the second series of brachials, Betabrachs, the 
upper or distal one of which gives off from the outer side an unbranched ramule, and from 
the inner the third series of brachials, Gammabrachs; and so on alternately — a ramule and 
then a continuation of the main arm, until the latter terminates with an equal bifurcation. 
The outer ramules of each axil-arm always tend to be larger than the inner, as explained 
by Bather, so that the first ramule on the extreme outer, or adanal, side, with the Betabrachs 
that support it, becomes the most conspicuous branch of the arm, while the rest of the arm 
is correspondingly diminished and sometimes completely hidden from view. Thus it results 
that the Alphabrachs, the outer Betabrachs, and the outer ramule appear to form a continuous 
arm, often all that is seen from the exterior, while the inner Betabrachs, Gammabrachs and 
the series which follow them, if any, although morphologically the main element of the axil- 
arm, are often inconspicuous or even invisible. In the closed condition of the arms as usually 
seen in the fossils, they are frequently folded under the adjoining outer ramules, while the 
latter constitute the visible axil-arms, lying parallel and gradually diminishing in length and 
thickness toward the anal side and stem. Especially the first or lowest ramule, given off by the 
axillary Alphabrach toward the median arm, is usually obscured in form C, but is more freely 
exposed in the later species of Form D. 
The axil-arm is a direct development from the equally divided, ramule-bearing arm of 
Form A, as may be seen by comparing it with the arms shown in figures 9, ii, and 13 of 
plate 28, in which the alternate ramules from about each second or third brachial reach the 
full height of the crown — the essential’ difference being that in the earlier form both rami of 
each arm branched that way, and the main branch maintained its preponderance over the 
ramules. From the symmetrical ray ol Form A, v;ith its two equal and similar rami composed 
of successive ramule-bearing brachial series, and dividing upon an equal-faced primibrach, to 
the fully developed axil-arm system of Form G, borne upon an unequal-faced primibrach, 
with nothing of the original arm-divisions left visible but a set of closely packed parallel 
7 
