34 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
arms proper; but when smaller and shorter than the main arms, they are 
called armlets. If there are small lateral appendages, given off alternately 
from opposite sides of the arms, they receive the name pinnules. The arms 
are uniserial when their joints extend through to both sides of the arm; 
biserial when they do not, but interlock from opposite sides. 
The brachials succeeding the radials (the first axillary included), whether 
free or fixed, are called costals, or primary brachials; those of the second 
order distichals, or secondary brachials; those of a third order palmars; and 
all succeeding brachials, whether there are additional divisions in the ray 
or not, receive the name post-palmars. When in the description of a species 
it is necessary to specify any of these plates, they are distinguished as 
brachials of the fourth, fifth, or sixth order, and so on to the last bifurcation. 
We also find it convenient occasionally to refer to them as the plates be- 
yond the fourth, fifth, or sixth axillary ; or, when free, as plates of the first, 
second, or third division of the arms. The plates of the different orders, 
according to their rank, are distinguished as first, second, or third costals, 
distichals, palmars, ete., and the bifureating plates as the azidlaries of their 
respective orders. All these appellations, however, are not applied to the 
divisions formed by the armlets and pinnules, although the plates which 
support them are in fact axillary, and each armlet or pinnule is morpho- 
logically the homologue of a whole dichotom. 
When two or more arm joints meet transversely by a rigid suture, and 
only the upper one is pinnule-bearing, those joints form a syzygy, whether 
the apposed faces are radiated, dotted, or smooth; the lower joint bearing 
no pinnule is called the hypozygal joint, the upper one the epizygal. 
The spaces between the rays and their subdivisions are filled by supple- 
mentary plates. Those between the rays proper are designated by the 
general term iterradials, whether they belong to the dorsal cup or to 
the ventral disk. Those of the dorsal cup, which are interposed between the 
brachials, are distinguished as interbrachials, and those of the ventral disk, 
which lie between the ambulacra, as inferambulacrals. Plates between the 
radials at all five sides are only found in dicyclic Crinoids, but in most of 
the Palwsozoic Crinoids there are one or more such plates at the posterior 
side — the so-called anal plates. 
The anal plates form the base of the anal structures, and consist of the 
special or first anal plate, which, when present, invariably rests upon the 
truneated upper face of the posterior basal, and between two radials. Most 
