34 THE CEmOIDEA CAMEEATA OF NOETH AMEEICA. 



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 

 ^ve uniserial when their joints extend through to both sides of the arm; 

 hiserial 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 palmar s ; and 

 all succeeding brachials, whether there are additional divisions in the ray 

 or not, receive the nsime posl-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, etc., and the bifurcating plates as the axillaries 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 syzygij, whether 

 the apposed faces are radiated, dotted, or smooth; the lower joint bearino" 

 no pinnule is called the liypozygal ]o\x\i^ 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 interradials, 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 mterlrachials^ and those of the ventral disk, 

 which lie between the ambulacra, as interamhdacrals. Plates between the 

 radials at all five sides are only found in dicyclic Crinoids, but in most of 

 the Palaeozoic 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 

 truncated upper face of the posterior basal, and between two radials. Most 



