MORPHOLOaiCAL PART. 105 



III. 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- 

 palmarsj 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 Acrocrinidae, 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 Palgeozoic 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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