MORPHOLOGICAL PART. 105 
Ill. SUPPLEMENTARY PLATES. 
A. The Distribution of the Plates, and their Relations in the Different Groups. 
The supplementary plates comprise all limestone particles between the 
basals and orals, and intervening between the rays and their subdivisions, 
They are divided into interradial, interaxillary, and anal plates. The inter- 
radial plates, comprise as a general term all plates between the basals and 
orals, interradially disposed. Some of them are distinguished as inter- 
brachials, others as interambulacrals. The interbrachials are confined to 
the dorsal cup. The interambulacrals occupy the spaces between the am- 
bulacra. The interaxillaries, consisting of the interdistichals and inter- 
palmars, are located within the axils of the second, third, and succeeding 
orders of brachials respectively. The anal plates are restricted to the 
posterior interradius supporting the anal tube. Another system of small 
plates occurs in the Acrocrinide, where they form a wide belt inter- 
mediate between the basals and radials. 
In nearly all Crinoids, recent and fossil, in which the free arms do not 
start directly from the radials, the lower arm joints are incorporated into the 
calyx, either by soft tissues or by means of plates. The latter are exceed- 
ingly variable in form and character, being in some groups well developed 
and rigid ; while in others they are irregular, ill-formed pieces, or mere lime- 
stone particles, resting within soft tissues. The great variation observable in 
the structure of the plates among different groups led to the belief that the 
rigid and regularly arranged pieces, which are so characteristic of the Came- 
rata, did not belong to the same system of plates as the irregular, small 
pieces which unite the rays of recent Crinoids; and Dr. P. H. Carpenter 
? 
applied to the former the term “calyx” interradials, as opposed to the 
interradial plates of the “disk.” 
A somewhat similar distinction was made respecting the plates which form 
the ventral pavement. The heavy, rigid pieces of Palaeozoic forms were 
called “vault” plates; and the small, irregular pieces of later and recent 
Crinoids, “ perisomic” or “disk” plates. The term “vault”? was generally 
applied in cases where the mouth and food grooves are permanently closed, 
and “ disk,” where the mouth and food grooves are open. 
In the Camerata, the interbrachials are nearly always arranged on a 
definite plan, and are stout, large, and united by close suture, making the 
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